I think most of us use "guess-timation" based on trial and error. When I started, I mixed smaller batches (which typically gives you a bit more working time at normal temperatures) and found that it usually went somewhat further than expected.

Using West Systems pumps (5:1 ratio), for scarf joints, I typically used a single pump of resin and hardner. For the laminated deck beams, I think I used two pumps of each plus thickener (always mix the epoxy first and then add thickner). For the first "wetting out" coat of epoxy on the fiberglass bottom, I used two batches of six or eight pumps each. Subsequent coats which aren't soaking into the bare wood or glass use quite a bit less epoxy.

BTW, both West Systems and System Three have lots of excellent "how to" literature either free or very cheap (some of it on their respective web sites). I found it very informative in learning about the various fillers, hardners, temperature considerations, etc. This knowledge can be gained fairly quickly and is MUCH cheaper than repairing a botched job.

Good luck!