Rock tumbler grit works well for grit, as it is usually silicon carbide. The grit I used was a little coarse, which has cause minor focusing issues, as the grain in the glass can be quite large, though it is much brighter than the "factory" glass.
One tip, I prefer to grind the glass before cutting, as this reduces the problem of un even grinding in the corners. For a 4x5 screen, I start with about 6 x 7 or 8, and then cut down. Cutting glass is quite easy once you get the hang of it, and it's a $9 tool and a bottle of baby oil.
The bottom of a beer bottle works well as a grinding tool, and has a familiar grip to it.