I dropped this in with the new field. It seems to work for the second condition but not quite for the the first.
So with the adjusted fields:
Field 1 ($ Charge) = $1.50
Field 2 [Currency) = AUD
Field 3 [FX Rate] = blank
Field 4 [$AUD Charge] = should therefore be the value of Field 1 - $1.50. But with your formula it seems to be showing as 0