ATM a couple fine grinding questions

I am fine grinding a 12.5" pyrex blank using a full size ceramic tile tool. I'm currently using 220 carbo and find that there is great resistance when grinding MOT. When i grind TOT you can tell that the grit is really working quite nicely, and there isn't much resistance, unfortunally I'm lengthing the focal length too much and need to shorten the focal length by grinding MOT. When i throw down more grit and water the grinding is easier and smoother for a few moments until the grit ends up in the channels between the tiles and then the resistance continuely increases until I can hardly move the mirror. It gets easier again if i throw down more grit and water. At this rate I'm going through an incredible amount of carbo.

Also, after evaluating the progress that I've made I notices very, fine scratches over the surface of the mirror. They are difficult to see but under direct sunlight you can see them easily. Could this be due to pushing the glass with enough force to over come the resistance on the ceramic tool with little grit on the surface? I'm useing very clean technique (no contamination) and the ceramic tile is WB hex tile. Will these fine scratches cause me troubles? Thanks for the suggestions.