When I booked this originally for this year it was back from Narita and I voiced the mileage concern to the BA team when booking but was surprised when they intimated that the mileage calculation was not something they needed to factor.

That​ said when BA moved to Haneda and I knew I had to lift and shift to 2021 I did the calculation again for my own comfort and came to the same conclusion that it was close but still ok.