This is because if you add more FASA then needed, there is no way for the excess fumaric acid to precipitate out along with the alks that your extracting. Its completely soluble in the acetone and will stay in the acetone, while the fumarates on the other hand crash out. A FOAF was going to try this soon and was going to just add either the same amount of FASA that there is CASA, or maybe 1.5 to 2 times the amount. Like I said, the fumaric acid wont precip out, and this will just ensure there is enough to completely convert every alk to its fumarate salt.