As you identify, this is too close to call and it's always best not to rely on independent Mileage calculators. GCMap, incidentally, gives a slightly higher result of 959 miles for your route, and 972 miles for the PAR metro area - which A3 sometimes uses, certainly for London (albeit they fly from both LHR and LGW) - so this might give you some more comfort.

To counter that, A3 has been known to award different miles for the same route with different airlines, so it's fairly unpredictable!

What might be a bigger issue is that past experience is not necessarily a guide to future experience, so even if FT'ers have flown this route in the past the information still might not be entirely accurate - I've had SQ flights from LHR-SIN where the mileage awarded has changed by about 1% over the years. Not enormous, but potentially enough in your case if it goes the wrong way.

Your best bet is probably to call A3 and ask them what the earnings would be - others have done this with success. That would, at least, give you today's figures.