Pre-hogging work

I have done approx 3hours of grinding on my 12" mirror. The central depression on the tool has been reduced greatly, though not eliminated completely. There is a central circular area on the mirror, approx 4" diameter, that is not in complete contact with the tool. I have been able to decipher this because of three tests. First, I put a piece of printed paper, having about 12-15 font size on a flat surface. I rest the mirror on the top with the grinding surface facing up.I try to read the material printed on the paper. I cannot read the material from anywhere except, from a central circle about 4" diameter. Secondly, this central circle also does not show any pits/scratches while the remaining mirror surface shows almost uniform sized pits. This means that the central area is barely getting ground while the remaining surface is getting abraded. Lastly, I used the sharpie test. Let me describe this test in brief.

Before doing this test, it is advisable to completely wash and dry the mirror surface. In this test, one draws a series of parallel lines 1-2" apart on the mirror using a graphite pencil. This is followed by another set of lines, but perpendicular to original set of lines. Thus, the mirror surface will resemble a series of squares. After the lines are drawn, one proceeds with the grinding. While using a less abrasive grit (such as #220 or #320) is preferable, it can also be done with the grit size you are using. I continued with the #36 grit that I have been using. I did about three wets i.e. added the powder charge and water three times and grinding the two surfaces each time till the abrasive breaks down and I have to add a new charge. After doing this, the mirror is again washed. Just allow water to flow over the mirror and try to clean the mirror surface without touching it. Gently use a very soft cloth to clear out the abrasive only if it does not wash away with the water alone. Again allow the mirror surface to dry out completely and then look for the pencil lines. The lines should either have completely disappeared or have uniformly faded. Both these scenarios indicate a proper and uniform contact between the tool and the mirror. In case of my mirror, the lines in the central 4" diameter circle were just lightly faded while lines in most other areas disappered. This just confirms my earlier observations. What this means is that, I will have to do 2-3 more sessions with #36 grit to get complete contact. Only after this will the actual hogging i.e. hollowing out the mirror can start. I am estimating that I should be able to start hogging next week.