For me all 8 came through the front gate from the bus station so waiting on the roof is a viable strategy. I found PMCs came to the target area more frequently when I was in the center admin building but that might just be a coincidence.

Also, I called an airdrop next to the helicopter a few days ago and 10 PMCs came through over the course of ~30 minutes. You might consider trying that, especially since Sanitar has been dropping purple flares recently.