Hey Nick
on my D5 Next 2500 rpm is 66 liters.
I also have the problem that my HighFlow Next shows 0 liters at system start if the flow is below 80l/h.
Only when the pump delivers higher speeds for a short time (increase flow) does the HighFlow display values again.
I've had my pump run at 85% since that's where the noise bothers me the least.
Faster and slower the frequencies are too annoying for me personally