Mid to late summer the Rock is not a very good gauge of what is happening on Filter Plant as there are two diversions below the Rock and above Filter Plant put-in. The flow on Filter really depends on how much is being diverted and how much (if any) is coming in from the North Fork/Seaman.
As Lebowski noted, flow tolerance depends on what you're floating in. I've run it with my kids at very low flows late summer and early fall (under 200 cfs) though I seem to scrape over every possible rock while my kids breeze through in IKs and hardshell kayaks.
If Filter is too low and you're looking for an easy run, go up and scout the lower mile or two of Lower Mish, down as far as Hewlett bridge. There are a couple day use areas where you can put in below Ansel Watrous and avoid most of the rapids.