It is very, very difficult to predict the transient flows in a system. We say at least 25% above maximum normal system flow and should say 50%. Don’t back yourself into a corner by sizing a flow fuse at the top end of its range unless you can test it. If you have an FQCA in a 4 gpm (15 L/min.) circuit, set at 6 gpm (23 L/min.) and it false trips, you are out of luck. Select an FQEA and leave yourself some room for tuning.