From what I understand your system should cycle through the volume of your fish tank in an hour. So you would need to find the volume of water in your plant bed not the total volume (subtract the volume of gravel) and figure out how many plant beds fills equals the volume of your fish tank, then figure out how long it takes to fill the planter and how long it takes to drain the planter. From there you can calculate if you need to fill the planter faster or slower.I hope this helps.
Alternatively a stopwatch and a 5gallon bucket does a good job at measuring flow rate. The tank volume through the filter once and hour is a good rule of thumb but a less I wouldn't sweat over and the flowrate is likely too high if it is more. Better to put in a T fitting and a ball value and just bleed off the excess directly back to the fish tank which will help with aeration if you need to reduce the rate. More importantly it will allow you to adjust the flow to make sure your siphon is working correctly which is more important, if you are using a siphon that is.