Well.
My gc2b came in the mail today and I couldn't even get the darn thing over my head. (Almost got stuck, as in, HELP, I am home alone with no shirt and a binder stuck halfway on my head because I'm trying to secretly crossdress, and can't even utilize scissors in this position!) So obviously I need to send that one back, the (rhetorical) question is should I go one size up or two.
Some more of my guy clothes came in the mail today, a different brand, and ironically those shirts looked fine with the other binder.
So I think this is all a MASSIVE amount of trial and error.
Unrelated to binders, but I ordered two pairs of pants from the same brand in the same size - one fit perfectly, and I couldn't get the other one over my butt because of how the pants were cut.
Everything that is too small for me fits my best friend, so I am donating everything that doesn't fit me to him

He's a champ at listening to me whine about my life.