My Receipts report has seven currency fields, each of which uses a variation on =IIf([code]="A",[Amount]*0.4,Null)
The Total field has the formula: nz([DonationA1],0+nz([DonationA2],0+nz([DonationB],0))) (shortened from seven fields).
All but one of the six fields is usually Null, (depending on...