I understand that they want to sell what they have. But when they tell me it fits perfectly, even when it fits like a circus tent and I show them how there's a six-inch gap between the jacket and my stomach, well, there's no excuse for that. Maybe some of you are used to being told what fit you like, but I have a pretty well refined opinion of what looks good on me. A sales pitch isn't likely to change that. I'd have lot more respect for a salesman who told me he had nothing that meets my needs. I might buy shirts and ties from him, or pants, which are simpler to fit correctly. But if he can't tell a suit jacket from a Ringling Brothers big-top, I'm walking.
A 6" gap is normal. You just don't know what's normal and have likely not worn suits very long.
Shirts should also measure 6-8" larger than your chest
Knit shirt 4" larger than chest measurements.