I keep a rack of try-on suits made up in my house pattern, scaled to fit a variety of sizes. We’ll find one that fits well, and start making adjustments from there. I like to think of this as equivalent to a first fitting-- I’ll pin, mark, and measure, and you’ll start to get a sense of how things are going to look and feel.
I’ll make note of all these adjustments, and they’ll be used to make your new pattern. I’ll adjust for everything ranging from the slope of your shoulders and posture, to the pitch of each of your arms. I do the same thing here as I do during any bespoke fitting.