The easy way to work out how much glass to put into a mould is to measure the volume of the mould. (Tip: whether you normally work in litres or pints, this is MUCH easier of you work in litres.)
Fill the mould with water, from a jug, so that you can record how many litres it takes.
OR: put the mould on some scales. Zero the scales, then fill the mould with water. The mass (weight) of water in kilograms is equal to the number of litres that the mould holds.
Multiply the volume in litres by 2.5 (the density of Bullseye glass) to give the number of kilogrammes of glass needed. This weight of glass will be the same whether you're using frit, sheet glass, casting cullet or billets (but frit will take up a lot more space).