Just for the fly spargers out there...
I've tried slowing down my sparge to meet 1 pint/minute. It occurred to me that if I bother figuring this out, I could probably stop watching the kettle for when to end the sparge. This actually seems to work. If I want 7.5 gallons for pre-boil, at 1 pt per minute, I'm done in 60 minutes.
My tried and true way was I had a wooden dowel (the other half from the '07 Flanders Red batch) and hashed out per gallons. This works well, except when I use a false bottom. That kind of messing things up, but still, the false bottom is at about the two gallon mark. Also, as the wort heats up, it expands and that can put a little hitch in readings. Probably the biggest though, is there's a degree of babysitting. Every sparge was different.
The way before that, I did total gravity (gravity * volume) to get to my desired gravity, and either kept sparging or added water to get to my pre-boil. This is an excellent system, but for some reason once in a while I'd mess this up. Really though, I lost interest in this when I finally stopped messing with my system and became reasonably predictable on the efficiency.
For a while I was calculating how much water I'd need, minus grain absorption and kettle/system loss. Then I'd just fill my hot liquor tank with that much water. When the HLT went try, I was done. This works well, but every once in a while for some reason I'll need more water. Kind of a pain when I have to choose between using straight tap water or filtering and treating a fresh bunch of water. Plus, I'm not too sure about draining the mash-tun dry. Kind of seems like I might be pulling more undesirable crud from the mash this way.
Yet another idea was sort of "pulse sparging" where it's sort of a hybrid between batch and fly sparging. I'd drain the mash-tun down a good ways, then refill with sparge water and keep going until I get what I need. This isn't a measuring technique like the last few. This is handy and seemed to offer up time savings without hurting efficiency. Similar as the pulse sparge though I'm not too thrilled with the idea of draining the MLT.
So, how do you fly sparge? My stop watch method seems to work well for me. Basically set it and forget it with really no room for instrument errors.