barit
On the -400, I recall it differently!
There is no thrust allowance for APU running at take off, in fact, if the APU is on for take off, there is a performance decrement due to the fact that the APU Door is open and adds drag! I think it is/was in the order of x00 kgs.
For that reason, the reduced fuel burn by not running the APU and better EGT margins, BA decided many years ago to go to packs off take offs below an arbitrary 300 tonnes TOW. Below that TOW we used a packs on take off technique.
Previously we had used 'APU to Centre Pack' take off techniques above 300 tonnes.