It should be simple: you simply align the flank feather on top of the hook while you're tying, measure it to get the length you want, & tie it down. Easy......well, almost. Usually I don't get the wing exactly right. Either it cocks to one side, or it isn't exactly straight with the hook. Mostly though, I have trouble getting the wing at the angle I want it at. Ideally the wing would be pointed at about a 45 degree angle from the hook shank. This is sometimes easier said than done.
The answer to my problems came in an article written by Eric Austin. In the article he shares a method he discovered from reading some old fly tying books. Well, what's old is new again & this trick has really helped me to get the wings right.
The trick is to tie the feather on upside down & pointed in the wrong direction (than what you're used to).
Please, take a moment to read Eric's article HERE. He goes into a lot more detail than I have & does a much better job of explaining it all than I have.
It's these little tips & ideas like this that take something that should be easy, but isn't, & makes your fly tying that much better. Thanks, Eric!!