If you have already pulled the bullets an dumped the powder in a container, this would probably be a moot suggestion, but if you haven't, here goes.
Pull anywhere from 5 to 10, weighing each powder load. Add all of the weights to get an average. Then, to be on the safe side, take that load and reduce it by 1 or two grains. If your powder throw is accurate and consistant, this should give you a safe load.
As RM said, though. If there was any chance of water intrusion, scrap the powder. Also, if the powder smells rank, ditch it and start fresh.