I've taken a stab at describing how I go about "dialing in" an espresso at various points in time and in various venues.
As time has gone by - I've refined my methodology and practice.
For what it's worth - here is my current model.
The idea: you have a coffee that is new to you, and you want to figure out the extraction parameters for this coffee that give you the best results (for your tastes).
The caveats: some of the stuff in here is specific for my own taste/perception/expectation of espresso. In particular, my concept of espresso is that it is a unique and different way of preparing coffee. In other words - I'm not trying to re-create the flavour of the coffee when cupped or brewed as drip or press-pot or something. I'm also not trying to create something that (as D Schomer put it - "tastes like ground coffee smells"). And I'm not trying to create some sort of HeMan beverage that "puts hair on your chest." For me - I'm trying to extract a balanced and complete mix of clearly defined and differentatiated flavours -- a round balance of sweet, sour, bitter -- that expresses the unique flavours and aromatics of the coffee. In addition, I pay for a small percentage of my beans and can thus afford to "waste" a lot of coffee. And finally, I work on equipment that is easy to "tune" and change quickly.
I try to start with some constants that are as likely as possible to be accurate for the majority of coffees. In this case, I start with an LM ridged double basket and a target volume of between 1.75 and 2.0 oz with a moderate flow rate (somewhere around 25 seconds for that volume, though I evaluate by flow not time).
I'll also try and understand up front the "signature taste" of the coffee.
The "signature taste" requires some knowledge of the roaster's style and the desired flavour profile of the coffee. Is the person roasting this coffee a fan of low acidity espresso? Are they a "chocolate bomb" aficionado? If you know what they like out of their espresso you can do minor adjustments to your dose. In many cases (these days) you can use the interwebs to research and find out what the roaster looks for. Otherwise, go by the retail location of the roaster and taste the shots.
This allows me to understand what I'm "shooting for" in the cup.
Once I have this information, I'll start making some guesses on the primary variables.
My initial goal is to make a quick estimate of likely dose volume.
I do this based upon the coffee (the bean/blend composition.)
If the coffee seems likely to have low pH (has robusta or aged coffees or a lot of naturals) I'll start with a down dose. If it seems likely to have a moderate pH (pulped naturals, a mix of naturals and washed coffees) I'll go with a moderate dose. If the coffee is high pH (mostly high-grown washed arabica) I'll up-dose.
This baseline is then slightly impacted by degree of roast (for a darker roast I'll drop the roast a percentage, for a lighter roast I'll up it).
On a side note - this focus on pH of coffee is new to me and is the result of reading the results of a research study done by Nestle Labs. This study found that coffee compaction in espresso brewing is dependent upon pH of brew water and pH of coffee. Coffee compacts under brew pressure - but according to this study, how much it compacts depends upon pH. Low pH coffees (robusta and aged or monsooned coffees at the extreme) compact the least and high pH coffees (high-grown washed arabicas) compact the most. Focusing on the pH of the coffee seems to have enabled me to more quickly estimate dose volume -- and this (so far) seems to my taste to be a very consistent system.
Anyway... once I have a starting point for dose, I'll try to come up with a starting point for brew temp.
I'll evaluate the coffee for two characteristics. First - roast degree and second - bean composition.
With the former, I tend to make some quick rough decisions. If the roast is light, I tend to start with a baseline temp of 202F. If medium, I will stick with 200F. If dark, I'll drop it down to 197F.
Now... I'll also adjust this based on the bean composition. If, for example, I'm working with high-grown washed arabica I'm going to reduce the brew temp. If I'm working with aged or monsooned coffees I will up the brew temp (both from the baseline above).
So a light roasted coffee with monsooned beans will move up to 203F as a starting point.
Once I've got temp and dose I dial in the grind and then I'll start experimenting.
I always start by re-evaluating temp. So I'll pull a shot and evaluate it for brew temp. Is it alkaloid? Is it thin? Is it sour or bitter? Astringent or burnt? Does it taste ashy or like fish oil? Based on the taste, I will alter the temp by small degrees to find the sweet spot. If it's sour - I'll take the temp up. If it's ashy, I'll take it down. If it's thin and lacks sweetness and fruit, I'll take it down. Etc. It's basic pattern recognition.
I'm not looking for a great shot here -- I'm just looking for the right brew temp. The idea is to get the balance of sweet, sour and bitter. If any of these dominate too much, I need to change brew temp.
Once I've found what I feel is the brew temp sweet spot, I'll start working on dose.
The way I tend to do this is focus on two things. First - clarity of flavour and second - roundness and balance.
If the cup is "muddied" I'll reduce the dose. If the cup isn't fully developed and sweet and rich I'll up the dose. Mouthfeel is one of the critical attributes I'm looking at here - as is sweetness and definition. If I need a "denser" cup I'll up the dose. If I need more sweetness, the dose goes up. If there is a lack of clarity in the cup, or it's out of balance (with the low end dominating and limited aromatics and high end) then I'll reduce the dose.
This is usually enough. It usually gets me to the point where I have a shot that I feel matches well with the signature profile and optimizes the coffee.
Now... that doesn't mean I like the shot. There are coffees that I just don't like. It means that I feel like I have a cup profile that fits the coffee.
From here I can "tune" with small changes in flow rate, volume of shot, etc.
This allows me to then tune the extraction for what I really want out of the coffee -- to force it a bit in my desired direction.
And that covers all but the edge cases.
But there are exceptions. There are these edge cases where all the above doesn't get me to where I want to be with the coffee.
It's usually only at this point that I start looking at changes to extraction volume and basket size.
For example, I've found that some lighter roasted delicate coffees tend to end up poorly developed no matter what I do - especially when they are pulped naturals. But if I then swap to a triple basket and deliberately down-dose (19 grams) I "open up" the coffee and it becomes more defined and clear. Or with monsooned coffee I find that the only way I can get the desired sweetness without getting a "wet cardboard" aftertaste is by going with a triple basket, normal dose and then pulling a ristretto shot. Sometimes I find that some high-grown washed coffees are best pulled very short and slow. The same is true of some Indonesians.
You get the idea.
I know that some people are simply not into this sort of fiddling.
And others find the lack of "science" associated with this approach to be problematic.
For me - however - this process and approach is not only successful, but quite enjoyable.
And most importantly, by making it entirely taste based - I learn from each and every coffee and each and every change. By not tying it to measurement and machinery but instead to what I taste in the cup - I learn.