Net of tax on all transaction types

Hello there, I've been asked by our Finance team to build a Saved Search showing all transaction counts for the month, with a drill down on a few details, i've used the formula {amount}-NVL({taxtotal},0) as this calculates net amount without tax correctly across both our UK and US subsidiaries, but noticed an anomaly, in that it correctly calculates net amount across all inward transactions such as Invoices, but on some outward transactions where a tax amount applies it adds these in. Conversely, if i use the Amount field it predictably adds tax to all inward payments to us, but excludes tax on some outward payments, i was wondering if anyone could suggest an adjustment to the formula above so that all transactions show as net of tax, so i can just have the one column? I've added screenshots of criteria and columns below, would be very grateful for any assistance, thanks.