It took me 28.5 grams of Fumaric acid to saturate 1000 ml of IPA @ 21.5 C.
One point where this is important to know is when you use FASI (FASIPA) to precip out D-Fumarate from NPS followed by re-disolution in H2O to separate out the remaining NPS and then dry. The idea is to use as exactly as possible only what you need. Otherwise, you are over-loading your final solution with fumaric acid which, on drying, throws off the final product weight and purrity with excess Fumaric. Of course, after dried you can wash that out with IPA but then you lose some D-Fum also and add another step not otherwise necesssary. (You can also do the precip drip by drip until no more precip forms, but on 500 HRRB batches, this takes a long time. I just dump in the correct amount of FASIPA, let the solution clear (15 mins) and then double check with a single drip.
By the way, I have found the MAX ION tech plus this FASIPA the best overall method with yields approaching 3%.