I don't have my reactor yet (just ordered today) but I think its safe to assume the flow would totally depend on the pump use and amount of stones.
Also I have a feeling the flow will probably slow down after a few days once the rubble settles to the bottom of the tray restricting flow a little.
I'd make sure to use a pump with adequate power to feed the reactor then adjust as needed with the valve. Also I am not sure how your system is setup, but I would not use the return pump to feed the reactor unless the return pump is strong enough and provides enough flow. What pump are you using and how big is the system?