OK, sport. The "problem" with your method is that it ignores the water that's in most toilets goes into the overflow tube.
You can account for this by:
1) Flush the toilet normally and time how long the float valve permits water to flow. 2) Flush again but hold the flapper valve open and cause the water that goes into the overflow tube to go into another contain. Let that water flow for the time arrived at in #1, above. 3) Add water that would have gone down the overflow tube to your base measurement.