I find the angle to sharpen at by drawing the blade across a strip of leather. Start at a very small angle and keep raising it a little until you find the angle at which the blade starts to dig in, or cut the leather. Then I just mimic that angle on my stone.

Also, after you have sharpened a knife a few times you have to sharpen it at a smaller angle to grind the edge back a little. After you grind the edge back, you can sharpen at the original angle.

I hope some of this made sense.