The best method I've found is to print a full scale image that you're fairly familiar with printing getting it to look exactly like you want it to when wet, set this aside in a tray of clean water, then, make several subsequent prints backing off the time by some increment; say, maybe, 2 - 3%. Dry all test prints as you normally would and compare to the wet one--whichever matches wins. Don't worry about the low values or overall tonal range...the high values up to and including Zone IX are the ones you'll be looking at. When you find the dry print that matches the wet one you've got your dry-down percentage.
All that said, though, you'll find that this percentage doesn't always work for all images. Many times I back off just a bit more and, when dry, the print really sings! If you happen to have one of the old Zone VI cold light controllers, you simply dial in the percentage and print away.