Oatmeal Stout style carbonation range is 1.9 - 2.5 vols. I used about 4.5 oz dextrose when bottle conditioning my Oatmeal Stout for about 2.2 vols. That's a little over 3/4 cup of priming sugar.
Never had good experiences when priming with DME. Always dropped a little more "junk" into the bottom of the bottles.
Of course, now I force carbonate and only bottle condition "special" batches that need to age for a while.