The other way is to test with a Hindle sphere. This is merely a spherical mirror with a hole in the middle so you can test the convex surface. There are a fair number of hits for "Hindle sphere" on the web and one may have the necessary formulas to make the distances easy to do.
The Hindle Sphere Test for convex mirrors is basically a test where you move the Hindle sphere to a point where it reflects the proper reflections of the secondary glass back to the tester. It is a null test when done right so all you need is a good Foucault Tester with the light coming from the same position as the eye (use a beamsplitter cube to do that nicely) and the secondary will completely gray out all at the same time.