I, too, like the one hour rule. Since I run off 7 gallons into my brew kettle in 1 hour, this equates to 1.17 gallons every 10 minutes. I have a mark at 2 gallons, 4 gallon and 6 gallons in my kettle to gauge volumes. Know the volumes at certain points in your kettle (like a valve, rivet, etc.) and you can keep a pretty accurate estimate of how fast you are flowing into the boil kettle.