As far as pilot FCOMs are concerned there are only a couple of F.Ctl inputs to the ADIRS system. ELACs and the BSCU. Given that it should not (it is Airbus after all) be ELAC it could well be the BSCU. Agreed it would be a very coarse way of measuring the input required to maintain track but it would appear to be all we have. Logically by the time we reach 100 knots we should have the thing vaguely under control so a measurement about then would be roughly correct.
Logically that is and as I said it is Airbus.