Hi Deceived - The material consists of "solids" or aggregate, resin "liquid" and air. If you do the water test you will establish the solids ratio. This can be 60-80% by volume depending on what your doing. Ratios by weight are convenient for making stuff but you have to work in volume ratios to figure out what's happening at the material level. Then convert to weight ratios for the actual mixing. So say you figure out your aggregate is 70% the solid volume of the mould. Then since your using quartz the attached calculation shows you have 13% by volume air in there if you use a 10% addition rate of epoxy. Now 13% by vol of air in that 77kg mix is 6 grams so you can't pick that up on a kitchen scale but you will see it in the mix as bubbles. To remove that amount of air you have to deep degas the mix which is difficult to do. Vibration and warmth may allow a lot to rise to the surface. Unless you do test mixes and check carefully you never pick up the air by weight... cheers Peter check the math I have been wrong before...

what usually happens is moulds are stacked by mixing small batches and they fill batch by batch until done. if they mixed the "correct" amount they would find they have too much by about 10% volume...due to the air in there.