Need help, grinding 12.5

My 12.5 mirror is ground to about 3/8 from the edge. That gives me about a 11.75 inch convex. The sun test measures about 67 inches which gives me a f/5.7. I want a f/7 or f/8. I am using a 10 inch tool. I have been grinding TOT and the convex is reaching the edge, but the f ratio is staying the same. What can I do to flatten the convex or lengthen the f ratio? Stroke? long, short? how far over the edge? Thanks, DaveT

Dave,
Since your tool is only 10 inches you'll need to use a longer stroke than normal to make it change. A normal stroke may do nothing to change the radius. Don't use too long a stroke though.
Ed