how much space to leave on a ssd?

It depends on whether the SSD is also carrying the operating system (Windows) and swap files (virtual memory). Windows needs enough space to maneuver itself and manage its own temp files, then so do the games when they are actually running. Running Win 10 with less than 25-35 GB free (certainly less than 20 GB) would begin to make me nervous. You could probably cut things a bit closer if you have multiple drives and this one is only running the games.

But regardless, 85 GB of free space is plenty.