A minimum of 44 degrees off of vertical seems to work for most people. I actually prefer a longer angle but that is difficult to find today.
Many factors come into play when choosing a string angle that fits you the best, primarily though is the use of facial reference points such as the tip of the nose and the corner of the mouth and those will vary from individual to individual. The structure of your face will affect the angle necessary to achieve these points as well as the length of your neck and shoulder position.
Many of today's archers try to use these reference points with varied success because of the compromises they make to reach these traditional references. In most cases they end up shooting too long of a draw, too high of an anchor, raising their shoulders, ducking their head into the string and many other form breaks in the effort to reach the string with their nose.
Axle to axle length, cam/idler diameter, brace height, limb tip geometry and deflection plus draw length all affect the effective string angle and thus the fit of the bow to an individual. But don't neglect the body build either, just because someone prefers one string angle to another doesn't mean it is the best for you.
The peep position makes a lot of difference in the way I shoot. A lot of it I don't understand, I get there by trail and error. If I have my peep in a low position it feels very comfortable and when I come to full draw my peep is in a perfect postion for my eye. When it is in this postion I don't shoot as well and I do when the peep is a little higher and my head is up.----A couple weeks ago I changed strings and cables and and realized my axle to axle was a long, after shortening my axle to axle I guess it changed my peep angle and that screwed me up for awhile.---- Javi should publish a book on what he knows. Mike has more useful information than Larry Wise and he has a book that sold very well and it contained a lot of BS.---If Mike publishes an Archers Manual, I would like to be an investor.