You could make a cardboard cutout of the shape you want and match it to the wing as you sand it to shape. A sanding bar (@3' of a flat material with sandpaper glued to it) helps in getting the edge even. Hold a straight edge (ruler) perpendicular to the leading edge and look at where it contacts the leading edge, you can see if its centered on the edge and how you need to sand to shape.

Good luck.