One thing I did working on my cowl was to cut a piece of plywood about 8"x24" and bolted it onto the prop spacer where the prop was going to be. I recessed it about 1/2" so it gave me a clearance between the finished front edge of the cowl and the prop. That gave me a good plane to measure from to rough cut the cowl.
If I build again, I will make the top half as big as I can lowering the hinge as far onto the lower piece to make the lower cowl easier to take off.