Just to throw it in the mix, I program GM engines at work for boat and industrial use, every engine requires a different spark map to be optimal. One engine tops at 15 degrees at 4500rpm, but is 35 degrees at that speed and lower load, others are in the 25 degree range at full load/speed. The timing tables I use are a little fancier than just a curve though, they change with manifold pressure and speed instead of just speed. For a plane, a prop load curve is close enough to just use a curve though.