well for making FASA you really dont have to measure.. just add, say, a teaspoon of fumaric acid to, say, 100ml of acetone, and mix very well.. If there's undissolved fumaric acid on the bottom, it means its saturated, so youre good to go just decant the FASA away from bottom fumaric acid.. If it all dissolved, it means its not saturated, so add more fumaric acid. And as for quantity of adding FASA to the bufo-laden acetone, just add till it stops changing colour. After retrieving the bufo fumarate, you can add some more FASA to the water to see if it clouds up again (meaning theres more bufo fumarates there). Sorry for not being so exact but if you understand the principles there's really no need to be exact.
Btw, remember to mix the bufo fumarate and the sodium carbonate BEFORE adding the water. I once did the mistake of adding the water before it was well mixed and the fumarate+sodium carb formed a taffy/resin-like substance that screwed up my extraction