In honesty there is right or wrong flow rate. Each job may require higher or lower flow. Also each system is set up slightly differently to suit the person using it. As a general rule of thumb Flow between 30 - 65 should be ample for most circumstances. Although on occasion flow of 99 for rinsing may be useful.
In some cases simply running the pump faster to push more water in to the hose line can reduce the flow at the brush because of reverse water pressure created by the restriction of the hose.
these two articles touch on this
http://www.springltd.co/node/138 &
http://www.springltd.co/node/137The 1st looks at pressure dynamics and the second flow.
One purpose of a controller is to give flexibility of flow rates as needed, while allowing you to use the minimum amount of water, battery power, required for that particular job. At the same time the controller reduces strain on the pump motor, and line connectors as we can reduce pressure in the system without impacting on the flow.
Duncan if you are looking for the ability to get a rinse boost have a look at the Wash - Rinse Ultimate RF controller
http://www.springltd.co/node/137Or of course you have the flexi flow version of the ultimate