I saw a topcoat I'd like to buy online, but I've never had any sort of overcoat before. The vendor measured the coat's shoulder and armpit width, and these correspond to the measurements of my favorite jacket, something made of polyester I put on against rain and wind. Would this be a good fit, or is a topcoat supposed to be even looser in the sense that it should be able to fit loosely over a jacket? I know a topcoat is supposed to fit nicely over a suit jacket, but I'm talking about a nylon/polyester type jacket for comparison where I've worn a denim jacket, a shirt, and an undershirt underneath. Is shoulder width a good basis for a topcoat's measurements?