I've never done this, but I think it should work (I said this in the welcome section already):
- You know your starting weight of caapi.
- You dissolve in some solvent until nearly 100% of alks are in solution.
- You decant and keep the solution, and evaporate solvent residue from the depleted caapi.
- You weigh the depleted caapi. You note the difference from starting weight.
- You subtract this difference from your target weight to get the amount of depleted caapi you need to put back into the solution for final evaporation.
However, I've since read that IPA isn't really good for caapi extraction. Water or vinegar should work better.
Then your harmalas will still be salts. For changa, you probably want them in freebase form.