Granted that similar to shoes, fitted hats do seem to have some variation on size. I've had 7 1/2's that fit right and others that fit a little on the tight side.
I agree with you in that having a reference based on hat sizes would be awesome. It seems like a pretty simple concept but I have no idea if it would be realistic.
Thanks for the info on the SC though. I have been considering an SC lid but may run into the same size issue as you. Depending on the source I wear either a 7 1/2 or 5/8 hat.