How can I get the subtotal of a foreign currency? I tried {fxamount}-{taxtotal} but that is subtracting the fxamount (in CAD) from the tax amount (in USD).

I think there is Amount (Net of Tax) field in saved search. You can use a formula to convert that to the transaction currency (amount net of tax internal id)/(exchange rate internal id. If you select None in the Consolidation Type field under the Results, all the amounts will be in FC so you won't need to convert. Not 100% sure since it's been a while since I needed this.