Putting in a manual entry will allow that to be shown on a report or printed map summary. That won't change your maps at all, just the total if you choose to show that manual entry. I'd guess you can't just change the total to change the entire map since all calibrations are different. You could have been right on for the high and average yield areas, but off a lot on the low flow. It wouldn't know that and have to change everything, including your correct data. 

Or....you can scale the data, which will take all of the data points (map) up or down by that percentage. Therefore your totals will change as well. Both strategies can be undone or hidden. Not rocket science or bringing said rocket back to earth.