Mirror grinding question

I'm in the rough grinding process of my 12.5 (60 grit). I am one inch from the edge. When I check the f ratio I get about a f6.3-6.9. I am using 2 methods. Sun check I get 67 inches, which calculates to f6.3, when I do a sagitta check, I get about f6.9 [F = r²/(4D)], D=. 095 r=5.25 (I’m only calculating the part of the mirror that has been ground, approx 10.5) which comes out to f6.9. My question is, when I go to parabolize, does the f ratio change? I want about f7-f8. I was told to go the depth of a dime and a penny (.110). Do I keep grinding with the 60 with the MOT until I get the .110? If I do, I’m afraid the f ratio will be to fast. My next class is Tuesday night, I was wanting to keep up with the 6-10 inch mirrors that are done with the rough grinding. I elected to do some grinding outside the class. I don’t want to have to go back and correct any errors so I’m asking the experts here. I am also using a 10 inch tool. Don't know if that is going to make any difference. Thanks, DaveT

I'm no expert, but I'd say if you are at f6.3 and you want to end up at f7, you should definitely start mixing in some TOT.

When you start parabolizing, the f ratio will change, but the total change in the radius of curvature is on the order of 1/32 of an inch, so this change is completely negligible.

Of greater concern, is the change in f ratio that can occur in the 120-220 grit range. On my current mirror (10 inch), I finished 60 grit at f6, but by the time I got to 320 grit I was down to f5.2 (I'm currently at f5.6-5.7 but that's a long story.) The other people in my class doing 10 inchers had similar results. I would guess that you will be at an even greater danger of this due to your undersized tool, so keep measuring your focal ratio until you get to 320 or so.