And I read it such that if you only have 6 months at entry, you must depart within 90 days of entry.
In any event, it is a silly risk to take as, if your plans change for good or bad reasons, e.g. sickness or injury, you are stuck with a visit to the US Consulate to obtain an emergency document (not that you need it for entry to the US, but more than likely, to board an aircraft).