Confusingly, I believe you only pay import duty on the actual item, but you pay VAT on the total.
So I take it you'll pay 20% of the Total inc. P&P. And then just a small amount for the Import Duty?
It's hard to figure out
exactly, because you don't know what exchange rates they're using.
Also, if something comes by normal post I don't think they charge VAT on the delivery charge, but if it comes by courier they do include the delivery charge. Warmoth use UPS, so you're paying VAT on the total cost including delivery.
I usually just estimate that I'm going to have to pay roughly 23-24% of the total cost, including the delivery charge. If it turns out less, it's a pleasant surprise.