Have you got Ronchi for Windows?
http://members.aol.com/RonWin20/
You can make a cardboard template if you print it out full size . I'd recommend a template of the outside edge of the centre two bands. Set a focus position that has plenty of bend on the outer zones. Just do an outside focus mask , and run a white board marker down the outsides and wipe off with metho after you've done your evaluation.
Cheers