The blade angle for shaving doesn't have too much to do with the bevel angle. There's a definite correlation, but for your average user, the angles are all the same. The blade should be slanted at about 30 degrees simply because at the level of sharpness that we are dealing with, if the angle is too low, you're more likely to be pulling the hairs instead of cutting them and if it's too high, you'll be scraping your skin.