Marcus you are correct. If you're making a purchase of any good outside the EU, this is known as an import for VAT classification. And should be featured in box 7 (Net Purchases) and no other boxes. Unless it's a service in which case it's slightly more complicated.
As Glenn mentions the best way to do this in QF is to mark purchases as "No Vat". Just ensure that there's a reference in the address / description on the purchase to show it's from the US - internal records only.
p.s. I should add, but not to complicate things, that if you have paid any duty or VAT on the import at customs (and only cross this bridge if you have) then you can reclaim the VAT element, but not the duty when you receive a C79 from HMRC. That is the evidence to reclaim the VAT back. Which means via a VAT return adjustment in the future you can directly increase the value of box 4 (purchase VAT) when the C79 arrives.
Thank you.