ladalet wrote: the variables you list are ... important and can affect shot volume. However they are more or less equally important (covary) to all machines independent of basket size. Basket size affects maximum dose size and makes machines unequal in expected shot volume with all of your listed variables being equal or optimized. Basket size places a limit to available shot volume.
I disagree with the above. The variables are
not equally important to all machines independent of basket size. The importance of those variables will be attenuated or intensified depending upon the basket volume in relation to its height. Two baskets with the same maximum absolute volume, but one of them broad and shallow and the other tall and narrow, will interact differently with, say, the same brew pressure. In isolation, maximum volume of the basket is a misleading metric.
ladalet wrote: Independent of the variables you have listed, a machine with a 49mm basket that holds a 11 gram dose will not extract the same volume of espresso as a machine with a 58mm portafilter that holds 18 grams.
I agree with this statement but this was not your original point. You wanted to give newbie baristas with a 49mm basket, say, or a 53mm basket, or a 58mm basket,
but not all of them, some metric whereby they could gauge whether they were making shots of appropriate volume for their basket.
ladalet wrote: And, if our understanding and expectation is that a double is 2oz of extracted espresso and the most that ones machine can produce is 1.5 oz of extracted espresso that one may reach the false conclusion that there is a problem such as channeling when attempting to extract that last .5 oz. There is not enough coffee in the basket to get that last .5 oz without blonding. There may be exceptions where the conditions may allow a 2oz extraction, but not consistantly.
OK. I certainly agree with you here. It would be unwise to expect all machines to adhere to some spurious ideal standard of volume. [It puzzles me that you are
seeking out deep baskets rather than accepting your own wisdom on this subject

] With lever machines, one should make espresso according to the machine's water draw rather than seek to produce shots that meet some conventional definition that has nothing to do with espresso
quality. To me that is as ridiculous as defining how large a piece of pie ought to be. Wide enough to stand up without falling over is good enough for me.
The amount of espresso produced will vary by machine type and even within type, both according to water draw and basket dimensions. Not only basket volume but basket
shape. Two baskets that hold the same volume but with dramatically different height-to-width ratios will not extract the same, even if the water-draw of the machines and their brew pressure happen to be identical.
Given these variations, espresso should be defined in terms of its extraction-intensity, which is what AndyS was trying to do, I thought, with brew ratios based on relative weights of ground coffee to water (though he did give rough volumes as a kind of handy reference).
Regards
Timo