True. Clerc gave for the green substance the formula (NH4)3 Fe2(Oxal)3, but the suppliers prefer to give the iron content (usually in the range 14.5-16%). Clerc's formula gives 15.24%.OTOH, I don't understand how those fine gram ratios are comparable since FAC is ill defined and composition is pretty variable -> differs according from supplier to supplier and even from batch to batch...
So, the ratios could be calculated also for the lower and higher values of that range, referred to the (presumabily) theoretical value. For example, 20:8 becomes 19:8 to 21:8 depending on if you have an iron poor or iron rich fe am citrate.