Some Math

Math to determine Sagita (depth at center of mirror).

Depth = radius (half the diameter) of the mirror squared, divided by center of curvature (Twice the focal length) X 2

For a 10 inch mirror with a focal point goal of 8 feet (96 inches).
Radius = 5, squared = 25 / twice the focal length (96 X 2 = 192) X 2 = 384
25 / 384 = 0.06510416666666666666666666666667, or just plain .065 for short...

For another example:

For a 10 inch mirror with a focal point goal of 6 feet (72 inches).
Radius = 5, squared = 25 / twice the focal length (76 X 2 = 152) X 2 = 304
25 / 304 = 0.08223684210526315789473684210526, or just plain .082 for short...