BA Avios flights: how to work out taxes?

[qpalbfy]

We plan to fly business class to the US using 150,000 points (2 people, using a companion voucher) how can I work out how much we will need to pay for taxes? Also how many points does it take for seat selection? Find the site really difficult to navigate so will likely call when we get to that stage but need to know what to save!

Ah I've actually managed to work the app and can see if we can get to 360,000 points it'll cost £900 total (does that sound right? I was expecting around £800pp?) At what point do you tell them you have a companion voucher? We haven't quite earned ours yet so system isn't letting me get very far and scared to commit myself to flights I don't want!