It says for amount over $250k the prevailing interest will be straight 3.5%. So there may not be need of any bonus interest to be credited in Sep.
I may be wrong, but this is my understanding (SSA):
Prevailing interest is calculated based on daily end-of-day balance. If you keep daily $250k or higher for the
entire month, you get 3.5% prevailing interest from the first dollar up. There will not be bonus interest.
If keep $80k daily (1st-10th), $180k daily (11th-20th) and $250k daily (21st-30th),
will get 1.5% prevailing interest from the first dollar up (1st-10th) - will have some bonus interest
will get 2.5% prevailing interest from the first dollar up (11th-20th) - will have some bonus interest
will get 3.5% prevailing interest from the first dollar up (21st-30th) - no bonus interest