What you need is a flowmeter not just a paddle type indicator if you want to know what the flow rate through the mash is. Flow rates above 1 gpm usally lead to compaction and flow problems, that is why I use a flow meter with max of 1 gpm. Sparge and runoff rates should be such that runoff and sparge times are at least 30 - 45 minutes for max efficiency. Coarsness of crush is a big factor to recirculation rates and efficiency, it is something that one has to work with to get a balance between flow and efficiency.