As I've never done this, I can only guess at the method. If you are using strips of glass, I'd lay them in the mold and fill up any spaces with chips of glass and then weigh the amount of glass used. Or carry out a trial run with some cheap glass previously weighed and see how much of the mold it fills up. Are you actually doing a pot melt and making a pattern bar to slice up subsequently? When I wanted to make a block of glass, I used kiln washed terracotta tiles for dams and let the pot melt spill into that. But you do need a large diamond saw to be able to cut up the block subsequently.
Guessing
As I've never done this, I can only guess at the method. If you are using strips of glass, I'd lay them in the mold and fill up any spaces with chips of glass and then weigh the amount of glass used. Or carry out a trial run with some cheap glass previously weighed and see how much of the mold it fills up. Are you actually doing a pot melt and making a pattern bar to slice up subsequently? When I wanted to make a block of glass, I used kiln washed terracotta tiles for dams and let the pot melt spill into that. But you do need a large diamond saw to be able to cut up the block subsequently.