Hi Grinders,
I have been using the same 22" glass tile tool for grinding 2 x 22" and 3 x 25" mirrors with varying f ratios. The second to last 25" ground was f4.28. For the last one of the 3, I wanted to produce an f5.0-f5.3 result. As the tile tool had too much reverse curvature, I decided to use my hogging tool on it to reduce the curvature. Using the same stroke, as I used to hog out the mirror, I was able to achieve the curvature reduction, and thereby, control the sagitta. At the end of the grind, (#800 finish), on the current 25"er, I have the Radius of Curvature at 258", or f5.16, for the 25" of clear aperture.
Note: Having experimented with both ceramic, and glass tile tools, I found that ceramic wears much too quickly, compared to the glass. Also the cutting action of glass on glass, is quicker than ceramic on glass.