Only speaking for wet fly wings, I prefer most of the time to use 2/3 rds the hook gap. Then other times depending on mood and creativity I use full gap, half gap and 1.5 the gap of the hook. When it comes to wet fly wings, there is no set standard. The width of the wing slips is best left up to the tyer and their likes at the time the fly is tied. Now you can look at paintings of wet flies in Bergman's book Trout & listen to Don Bastian preach that Bergman wet flies were 2/3 rds the gap of the hook. Far as paintings go this is correct and has a story behind it as well. Far as the actual flies this is Horse Hockey. Bergman's actual flies were all over the scales far as wing size, appearance and hackling. I use to be a follower and believer that a Bergman wet fly had to look this way or it is not so. After reasearch, investigation and then seeing numerous tyed Ray Bergman wet flies this myth was busted.

Sincerely
Andy Brasko