When I say, "mark a spot on the bucket," I mean you can choose to time for whatever quantity of water you want. Obviously even gallons are the easiest to compute. The more you measure, the more accurate the final number will be, you just have to adjust the equation. Multiply the number of gallons you're timing by 3600, then devide by the number of seconds it takes to get that much water in the bucket.
Example: You make a mark in your bucket at the three gallon level, and it takes 28 seconds to fill to that point.
3 (gallons) * 3600 (sec/hour) = 10800
10800 / 28 = 385 ghp (rounded off)