Mike is open March through mid November.
I've had good days at 1000cfs and lousy days at 300cfs. More to do with clarity, temp and hatches than flow. It usually fishes better a few days after a moderate spike in height. The lower park usually clears before the upper park.
This time a year, I like it under 500. I've had great dry fly action at 1000 but that's usually not the norm. .