Jay,
I'm the one responsible for the link that Ric just referenced. My epoxy balance has served me well for many years.
It was a project that started out as a “what if” sort of engineering project to see if it was possible to concoct a balance to accurately and conveniently dispense both hardener & resin in the correct ratio into the one cup. It has turned out to be a simple, repeatable, and error-minimizing way of metering the hardener & resin in the precise mix ratio it needs for the epoxy to work best. There is no counting pump strokes, no burping pumps, no calculations, and it is much more accurate than sighting to a fill-line in a container.
The balance arm is ~ 24 inches long. The the distances on the beam from pivot point to where the cups are hung for the adding of the resin and hardener are in proportion to the by-weight ratio of the particular epoxy that is to be used. Here's an extract from a long-defunct web page I had that showed it and described how it worked. Let me know if you want pictures or more info and I can see what I can come up with:

Here’s how it works:
I have been using 12 oz solo plastic cups for mixing my epoxy batches. I start the process by hanging a cup
A. Pour desired amount of Hardener in the outer cup (farthest right – labeled “Hardener”)
B. Balance the scale. Though not shown in the picture, I use a spring clamp for the adjustable counterbalance weight. I had a more elaborate sliding counterweight designed, but a spring clamp or two work better. I have a leveling line on the vertical guide that shows the balanced position where the beam is level -- adjust the counterweight to level the balance so that the beam lines up with the leveling line (ensure the counterweight is not moved from this position).
C. Switch cups. The empty cup gets moved to the outer hanger, and the cup with the hardener is moved to the hanger labeled “resin”.
D. Add resin until the scale re-balances. The scale’s sensitivity is such that is will begin to re-level in plenty of time to allow you to slow your pour rate as you approach the correct amount. (In the unlikely event that you slip and accidentally pour in too much resin, there is also an easy way to re-balance and add the correct amount of hardener to correct your mistake).
E. Grab your stir stick and start mixing.