If you're based in UK on a UK AOC, then your FTL scheme should be based on CAP371, which defines acclimatised as :
'Acclimatised'
When a crew member has spent 3 consecutive local nights on the ground within a time zone which is 2 hours wide, and is able to take uninterrupted nights sleep. The crew member will remain acclimatised thereafter until a duty period finishes at a place where local time differs by more than 2 hours from that at the point of departure.
and a Local Night is defined as:
'Local Night'
A period of 8 hours falling between 2200 and 0800 hours local time.
So before looking further into the "legalities" of your question, did you have 3 local nights in the US and hence were you "acclimatised" or "non-acclimatised"?