AK - out of curiosity, how do you get a proper fit, then, on your shirts? I don't see how a fitting for a bespoke shirt would work when the shirt the customer is wearing isn't really what it's going to fit like in 6th months. Does it just take a lot of experience to foresee that shrinkage?
Thought I cannot speak for other makers, 1% shrinkage allowance is left in our patterns. All of the cloth we use either shrinks 1% or is pre-treated by us in such a manner as to leave 1% shrinkage remaining. When we do our fittings, every sample shirt goes through a rigorous "torture" process of (multiple) hot washing and hot drying to insure that all shrinkage is removed before the client tries on the shirt. Thus, we know that what the final shirts will become after a few washes is what we are actually basing our trials upon. Regarding cheaper RTW and MTM, Manton is correct. The standard is to leave an extra 1/2". It is generally a good guideline.