Pick an easy to remember spot to place the clamp (most of the time, center of blade, from tip to guard, works pretty well, or right behind the nail groove on a folder), if you put it there every time, you'll be close enough to not degrade the existing edge. A millimeter one way or the other shouldn't throw you off enough to matter. While holding an exact angle through the life of the blade would be ideal, very little in life is ideal, and in this case, if you hold the same angle to within a degree or two, you should be able to get a VERY sharp edge if the metal in the blade is worth anything at all. Good luck, and remember... practice, practice, practice...

Troy