The best way to shape a leading edge is not with sandpaper but with a razor plane. Carving will give a much better result in lots less time than sanding and it's oh so much fun.

I use the Master Airscrew Balsa Plane. Run the full length of the LE on each stroke. It takes the wood off faster and with more accuracy and control. Once you get it close, then use a sanding block. You'll even find that you don't need a shaped sanding block.

What once was a chore is now one of my favorite things in building a plane. Unfortunately it works so quickly that the fun ends much too soon.

Dave