Depending upon whether you have gotten the glass precurved or not, you will have different needs.
For a flat piece of glass, consider about 2 lb. of 80 grit and 1/4 lb. of the rest of the grits wiht the possibility of 1/2lb. of 120 being desired.
For a pregenerated blank, 320 is a good place to start if the grinding was done with a good fine tool. It may take a few wets to get all of the marks out but that won't be difficult to do.
Scratch removal won't need you to go past the 25 micron size unless you really scratched the glass and those fine sizes of grit go a really long way due to their fineness.
As to pitch, better to get a 1kg. (2lb.) of the pitch so that you will have a suffiient amount of pitch on the base.