Not tried but my back of the envelope calculation runs like so:

Ferric oxalate = C6Fe2O12.5(H2O) mw 465 ish. 20% solution ~0.43M solution and so ~0.83M Fe
Iron(III) NH3 ox C6H12FeN3O12.3H2O mw 428ish 1M 20% solution ~0.46M solution and so ~0.46M Fe

so if its *just* the iron Id double my concentration. What the NH3 and oxalate do in the reaction I have NFI. Perhaps ask Mike Ware via his website?