Well, say you want to do a 30 fpm ascent from 75 feet to your 15 foot safety stop. 30 fpm is two seconds per foot, right? So you watch the gauge and you count, "One thousand one, one thousand two," and you should see the gauge click up one foot. Now, the gauge isn't that accurate, but neither is your count; if you get to 15 feet in less than two minutes by a little bit, you just stay there longer than your 3 feet (or you may catch things because you got to 45 feet too soon, so you stay there until the gauge clicks over the minute). None of this has to be perfectly precise. If you have a gauge with seconds, or a stopwatch, you can get really anal about it 
