I watched a well known gearbox builder rebuild Red Spit's. He had a bag full of syncro rings and must have spent half an hour just putting different ones together and feeling how they ... well, how they felt! He swapped them round, rejecting some instantly, keeping others and swapping them round until he was satisfied. Even then he would go back and try and improve on what he had.
My observation was, it's all in the feel and there's no substitute for experience!
(Not much help to you, I know)
Richard