Streamer fly hook size

I have a question about hook size and fly length. How do I know what hook size to use for a certain length streamer. Example, what is the longest fly that I can use a size 4 hook, all the way up to a 3/0 hook? I've tied a bunch of streamer flies for big fish and I want to make sure before I tie anymore that I know the right proportions. I was using 2-21/2 for 4,31/2-4 for a size 2, everything else is a blurry. I guess there is also alot a variables.

I know somebody will have a more precise formula, but, when in a quandry, we just tie'em on a tube and choose a hook later. http://www.flyanglersonline.com/bb/wink.gif
.....lee s.