it will go down evenly if you keep the pressure even and vary the pattern you work the head in. figure 8's, different direction circles, rotating the piece. it's not the space shuttle, dont fret too much.
also I second the emery cloth. its cloth backing will take the oil/lube/whatever without dissolving and buckling like regular sandpaper which isnt designed to get wet.
and... you dont have to do all 0.025 at once. do 0.005 or 0.010 and put it all back together, jet it, and see how it goes.
measuring isnt that hard either. just need a stable reference. have some digital calipers? before you start and as you work put the head down on a part of the glass/marble without anything on it and use the depth measuring part at the end and use the spark plug land as your upper and go through the hole to use the glass/marble as the lower. make sure the glass/marble and the part of the head youre taking down are clean and you will get as accurate as you need.
hope this helps. have fun