Proper ratios of epoxy resin and hardener are almost always specified by volume (for example, 2 parts resin to 1 part hardener; or 5 parts resin to 1 part hardener; etc). The mix can also be prescribed be weight. To convert from volume to mass, you need to know the densities of the components. The manufacturer surely does. MAS Epoxies provides a calculator for this purpose.

I don’t want to be handling my phone or a computer at the same time I am messing with epoxy, so here are formulae and tables for the mixes I might be using.

MAS Traditional Marine Resin + 520 Slow or 510 Fast Hardener by weight:

Hw = Rw * 0.18
Tw = Rw * 1.18

where Rw is the weight of the resin, Hw is the weight of the hardener, and Tw is the total weight of the mixture. Zero the weight of a mixing cup on the kitchen scale; pour in some resin and add hardener until the scale reports 1.18x the weight of the resin alone.

Likewise, when using Traditional Marine Resin and 320 Clear Hardener:

Hw = Rw * 0.30
Tw = Rw * 1.30

When using LV Resin and Slow Hardener:

Hw = Rw * 0.45
Tw = Rw * 1.45

Tables.

Traditional Marine Resin / 510 or 520 Hardener / Total:

50 / 9 / 59
100 / 18 / 118
150 / 27 / 177
200 / 36 / 236
300 / 54 / 354
400 / 72 / 472
500 / 90 / 590

Traditional Marine Resin / 320 Clear Hardener / Total:

50 / 15 / 65
100 / 30 / 130
150 / 45 / 195
200 / 60 / 260
300 / 90 / 390
400 / 120 / 520
500 / 150 / 650

Low Viscosity Resin and Slow Hardener (this is what was shipped with the kit in a former age of the Earth):

50 / 23 / 73
100 / 45 / 145
150 / 135 / 285
200 / 90 / 280
300 / 135 / 435
400 / 180 / 580

To measure these and much smaller quantities, I’m using small disposable cups and a small, digital kitchen scale covered with Saran Wrap to protect it in case of spills. Zero the scale to the weight of the cup, then add parts A and B until the numbers are right. Very little epoxy actually stays in the cup. The cure time is lengthy; the goop is still workable for 90 minutes or more at just over 60°F (is that a problem?).