The 60 day grace period following OPT end date does not care the 60 day grace period after graduation.  So even if your OPT started after 60 days from graduation, you will have 60 days grace period after OPT final date.

By the way u planning to go to other school or same school? Better try to get new I-20 within that grace period. If you get new I-20 before OPT end date, you should stop working right away.