First, I suggest that you simply leave the flow pulse rate as 1. This really doesn’t matter, it’s just a scaling factor. The volume in the end is the number of pulse times the flow pulse rate, so by leaving the pulse rate as 1, the volume you are reading in the end is simply the pulse count, you can multiply that by the correct pulse rate yourself. Again, this is really just a scaling factor.
The flow indicator you see on the screen always has a couple of seconds of delay, because the screen refreshes every second or so.
You should estimate roughly how many pulses the meter generates per second. If it generates a large number of pulse per second (by large, I mean more than 50 to 100 per second), then I am afraid it’s going to lose some clicks because the firmware uses a combination of interrupt and polling to handle flow sensor, so pulses that come too fast are not going to work very well. Maybe you should choose a flow meter that has lower pulse per gallon.