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Welcome to Flyertalk, GoingForGold1500.
The wiki has links to the Mennens formula where in conjunction with some searching you can work out the correct calculation. You need to include in your 75% for the carrier surcharge, but allow BA to the necessary taxes for WT travel, which from the USA will be all of them. If you dig out the e-ticket and break down the taxes (maybe estimating and looking at dummy bookings if necessary) you should be able to work out whether they got it approximately correct. WT to WTP can give low reimbursement, but usually that's in UK to USA rather than the reverse.