I completely agree that the fitter should account for changes in clothing, I just don't know how much.
Shooting SC in July with a t-shirt compared to a Pheasant hunt in January is going to be a huge difference. My K80 lop is 15.125", my field guns vary from 14.75" for early season doves to 14" for Jan/Feb quail. I was constantly getting hung up in my jacket on a covey rise with a 14.75" lop. I've learned over the years to change guns depending on weather.
I'm sure you're thinking about these kinds of things, hope it helps.
MC
