[nobby2 (Neil)]

I am using a second hand 8" mirror as a test. It came with some small chips in the back only and was silver coated.
I spent some time before starting, building my Foucault and Ronchi tester and have 3 gratings to use. I built a sagitta measuring tool out of a Dial Indicator and piece of bar stock.

The mirror started out as F/6.7 and Sagitta of 1.869mm. My aim is to get it to a F/4.5 requiring a Sagitta of 2.822mm.
Here are the lessons I have learned so far and the questions I still have.

I didn't have to use chemical removal of the coating as the first grind achieved that.
The Ronchi works fine while I had the high reflection of the silver coating but provides no reflection for a Ronchi test at least at the stage of using Carbon grit #220.
Question 1- Is this to be expected?
I was expecting to start using #80 or #120 however was sent a set of grits starting at #220.
I am currently achieving approx. 0.2mm glass removal per hour using the #220.
After three hours of grinding with #220 Grit I have increased the sagitta from 1.869mm to 2.33mm leaving 0.492mm to go. Possibly 3 hrs to go before progressing to the next grind. Question 2- Is this reasonable progress?