Measure epoxy on a postal scale. Most epoxy I use is a 50/50 mix. This also helps keep the 2 bottles level/equally used. I mix more than I need to make sure I do not run out, and to help rotate supplies. This 15-minute epoxy is over a year old. The mighty toothpick is one of my favorite mixer and applicator tool!
A couple of tips for measuring epoxy by weight:
1. Use a scale appropriate for the amount of epoxy you are measuring. If you want to measure only a few milliliters, consider a scale measuring in 1/100ths of a gram. That scale really usually only measures to 0.1 gms with ±0.05gm as the 1/100th unit. If you use a larger scale you can sometimes add a couple grams before the scale will even register.
2. Epoxy mixing instructions are almost always quoted by volume. If you measure Bob Smith Industries epoxies by weight, the SDS for all their epoxies lists a specific gravity of 0.97 for hardener and 1.15 for resin. That works out to a Hardener: Resin ratio of 1:1.185 by weight, or a Resin:Hardener ratio of 0.843:1.
The BSI FAQ says the weight ratio of all their epoxies is H:R 1:1.1 but the SDS sheets say different. BSI's opinion in the FAQ is that the hardener and resin need to be within 10% of each other and the 1:1.1 ratio complies (meaning they think you can measure 1:1 by weight and you'll be fine).
If you do it that way you will end up with leftover resin. Being a stingy, compulsive person not wanting to waste material I weigh it out at a 1:1.19 ratio and have not had any problems.
These ratios are for BSI epoxies only. I cannot speak for other brands.