> A spherical 8" mirror at F7 will have 2
> waves on the wavefront ( between the edge and centre rays )
Only if you insist on the parabola with the same paraxial radius of curvature ...
As you say, the closest parabola ("touching" at 70% zone) will depart only 1/4 wave (1/2 on the front). Texerau gives excellent description of all this stuff.
Only me being hyper-pedantic ... :-)
Bratislav