This sounds pretty normal to me... at the factory, they mentioned that the theoretical max MAP available is station pressure (or what the MAP gauge says with engine off) minus 1.0-1.5" for induction system inefficiencies. In practice, I've seen up to 0.5 below station pressure before the droop, every bird is different.
As for determining where that will be on a chart, the line graph form of the MAP limit chart that is available on the RHC checklist (available from the factory) seems to do better than interpolating the block chart. It's hard to find the max available MAP with solid accuracy on that chart. The other quick way is 29.92" (or 30.00" for rough calcs) minus an inch per 1000ft PA and then take off the 1.0-1.5" for the induction loss.
Or maybe I'm misunderstanding the issue...
Mike