How i can unemployed during OPT extension when i have already used grace period after graduation

I have completed my graduation in 14 Dec-2016, then i have received my OPT card in 17-Feb-2017 , in short i have used my grace period of 60 days. After 16 Feb i have remained unemployed for another 73 days during opt period. I got my opt extension from Feb-2018 to Feb-2020. Currently i am in Extension period and i am unemployed again. I just want to know how long i can be unemployed in F-1 OPT status. As per current rule for STEM OPT we can remain unemployed for 150 days. For OPT , clock of 150 days starts right after OPT start date or right after graduation???

in my case graduation i used 60 days grace + 73 days in unemployed in OPT period. Now i am in Extension, i google it says you get another 60 days during extention, request you to please clarify me .

Well, on paper it says you cannot accumulate 90 days for 12 month OPT and 60 days for STEM OPT period. As your situation is slightly tricky with grace period, I suggest you check with the DSO to get clarity or you can even setup an infopass appointment with USCIS and get clarity.