Easy answer: Dont mind quantities, just make couple hundred ml of FASA and add FASA dropwise slowly over some hours till it stops cloudying. Visual cue is very easy to follow, and adding excess FASA wont hurt your product so you dont have to worry..
The more complete answer.. It should be somewhat a function of fumaric acid solubility in acetone, the total amount of mescaline in the non-polar, and the composition of mescaline fumarate which I dont know (1 fumaric acid ion to 1 mescaline molecule or 1 fumaric acid to 2 mescaline , as is the case with DMT fumarate? )
So just playing around with some numbers... Let's say you are extracting 10kg san pedro/peruvianus/bridgesii, which can vary from practically nothing to 6g mescaline (
source). Lets say you are lucky and have the higher yielding cact and will have 6g mescaline total
Now for mescaline structure, Im unsure if its a 1:1 mesc:fumaric or 2:1 (like DMT), maybe a chemist can help, so lets try both possibilities. Here are the molecular weights:
116.07 g/mol fumaric acid
211.257 g/mol mescaline
so for every 211mg of mesc you need 116mg of fumaric acid at most, or 58mg fumaric acid if its 2:1 structure (unknown at this point).
So we can do an easy rule-of-3 calculation to see how much fumaric acid we need to salt 6g mescaline:
If salting 211mg mesc you need 116mg fumaric acid,
for salting 6000mg mesc you need X fumaric acid.
X = 116 * 6000 / 211 = 3299mg, or 3.3g fumaric acid
but very possibly mescaline fumarate has 2:1 mesc:fumaric acid ratio, so
x = 58 * 6000 / 211 = 1649mg or 1.6g fumaric acid
Now for solubility of fumaric acid in acetone. According to
this ,at room temp (20 celcius) 1.29g fumaric acid dissolves in 100g (
=127ml) of acetone. So to dissolve 1.6g, or 3.3g fumaric acid, you need to make
x = 127 * 1.6/ 1.29 = 157ml (if mesc fumarate = 1:1 ratio of fumaric acid to mesc) or
x = 127 * 3.3/ 1.29 = 325ml acetone (if mesc fumarate = 2:1 ratio)
If using Isopropanol instead of acetone, it dissolves fumaric acid almost 4x better according to
this publication so you'll need 1/4 of that volume.
TL : DR, if extracting 10kg high yielding cact you'll need at most 325ml 157ml FASA to salt out all your mescalineMost importantly, add that FASA very slowly to make purer crystals.