Should a chamfer affect sagita depth?

Hi guys!
I am finnally about to start grinding 203mm (8'') f/6 mirror. I got all equipment, built Focault knife and I am ready for hard work! According to Jean Texerau I have made 45 deg / 2mm chamfer on mirror edge.
My question is: should I take the chamfer into consideration while measuring sagita depth? This would reduce the mirror diameter to 199 mm (2mm chamfer on both edges) and change sagita depth required for f/6. But will the chamfer dissapear during grinding anyway?
Oh, and btw - what is the most convenient and precise way to measure sagita depth?

Hi Marcin

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No, if you do the math you'll see it has very small effect.

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The chamfer will reduce as you grind but a 2mm chamfer is quite a large amount to disappear. The chamfer is required to keep the edge from chipping out on the face of the mirror due to tiny stress points caused by variations in the size of grits. It also helps to prevent damage if you should accidentally knock the edge during routine handling. As you grind, make sure the chamfer does NOT disappear. You may have to touch it up a bit, much less as you move to smaller grits. IMHO, 1mm is lots during fine grinding and if it reduces to 0.5mm towards the final stages, so much the better.

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During rough griding, you can use a straight edge and a suitable piece of wire for a guage. You don't need precision. You can also get a good estimate of the focal length by measuring the distance to the best focus of an image of the sun. The wet glass will cast an image, poor but usable. When you get beyond the hogging out stage you can get a pretty good measurment using the image of the sun. If you want to get it really close, set the mirror up on the test stand and use the knife edge. Add a drop of dish soap to the water you wet the mirror with and it will stay wet long enough to make a good measurement. Don't struggle for perfection as the focal length will shorten a fair bit during the 120-400 grits. You can control the change by working TOT to lengthen, MOT to shorten.

have fun,
dan