Zone area can be adjusted to "recalibrate" usage estimates.
Here is more info on how it is calculated:
For example, if you find that your actual usage (as per water meter) is 40% more than reported, than the area for that zone needs to be increased by 60% (100% - 40%), in other words the final area needs to be 40% greater than initial area.
Alternatively if your actual usage is 40% less than reported, than area needs to be reduced by 40%.
Formula for this can be:
Final zone area = initial zone area / (estimated water usage / actual water usage)
or in other words
Final zone area = (initial zone area * actual water usage) / estimated water usage
P.S. by "usage" I mean change in reported gallons, long form of this formula may be:
final zone area = (initial zone area * (actual meter reading after the test - actual meter reading before the test)) / (estimated water usage after the test - estimated water usage before the test)
P.P.S. Duration of the test does not matter, but the longer you run the zone during the test, the better estimate will be. I'd recommend to run the test for at least 5 minutes. Calculations / adjustments need to be done separately for each zone.