Another mirror making a question

Hi All,
I am new to this mirror making thing so please forgive my explanation. I am grinding a 8" plate glass. My question is should my spherometer reading be almost the same across the mirror reading are 0.083 around the edges and 0.084 in the center of the mirror the reading are consistant . I was aiming for F6 which would have been a sagita of 0.079 ( oops ) misjudged my fine grinding.
Thanks ~~ Sonya~~

Hi Sonya, Glad to see you are pushing glass. You did not say what grit you are at so I assume you are still grinding and not yet polishing. I also am a newbie but am a little farther along than you at this point and here is my experience. The ATM Sag calculator gives a sag of .08333 for your mirror so you might want to give it the sun test. If your spherometer is a smaller diameter than your mirror the sag reading you get will be whatever 48inch focal length works out to for your spherometer diameter. I polishied a 10 inch F5 and got reading similar to yours. I saw a .001 measured difference until I got down to 12 micron grit, then I was able to get readings within about .0005. (estimated with a .001 micrometer) I never did get the same readings all over the face but after finishing 5 micron grit and polishing for 1 hour with cerox 1670 I tested with the foucault tester and was very pleased with the results. The surface was smooth and spherical leaning toward parabolic. I did my grinding and polishing on a machine. I got close with hand figuring but made a mistake which has required me go back and machine polish again. I hope to try my next attempt at figuring this week.

Good Luck with your project Dave