When using epoxy, pull out two paper towels and fold them up and put one under each bottle. After you measure out the resin, wipe the tip with the one under the resin bottle, then discard that towel. Measure out the hardener and use the towel under that bottle, then discard the towel. That way, you avoid cross-contaminating the resin and the hardener.
Also, when measuring 1:1 epoxy by weight, figure out which part flows slower out of the bottle. Measure that one last, as you can get closer to the right ratio as you're weighing it.
If you're measuring a high ratio epoxy, such as Aeropoxy PR2032, which is 100:27, measure the smaller component first, then calculate how much of the larger component you need and measure that.