The NGs APU will not reduce the pneumatic output as a result of an increasing EGT, when you're starting a main engine (Pneumatic priority). It will load monitor it's own electrical load to maintain air output for the engine start as the priority.
However, when you are operating both packs, say before engine start, an increasing APU EGT will result in reducing the air supply to retain full generator output capability (Electrical priority).
For me, if the engine N2 is accelerating normally as it progresses thru 25%, the start lever is selected up around 27%.
And since we have fuel-cooled IDG oil systems, I always look up at the fuel panel during the start lever selection (only for a moment) to notice the transient bright blue of both fuel valve indications, as well as no IDG spar fuel valve indication. When my eyes get back to the engine indication screen, it's usually just in time to see 'light-off'!
Works well for me...