As MSY is not a BA destination (requires a connecting domestic US flight) the calculator can't work it out for you origin to final destination only. Do a dummy booking to determine what your options are and then try the calculator segment by segment. LHR - DFW (BA), DFW - MSY (AA); or LHR - ORD (BA) ORD - MSY (AA). It is fare dependent (25%/100% miles/TPs).
I still have the bruises on my forehead from learning this the hard way

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