Working outside (the instructions in the ) box, there is a correlation between mass and volume:
density = mass/ volume.
If the density is uniform throughout the liquid in a bottle then a given volume will have a predictable mass. Half that volume with have half the mass. Each of the parts of the epoxy have different densities thus different masses per equal volumes. You can measure volume by weight.
My method also assumes that the two parts are at equal temperatures.
I am just saying that it is easier for me to do small amounts by weight than volumetric measure.