Use the average of all the grains in the mash. You have to consider them all (even the one already converted and unconvertible) because they all take up space. It's probably better to have it about 50 and that's not hard to do with a pound or two of American base malts. Lower DP may take more time for the same fermentability. I've seen a lot of old British recipes with a small percentage of six row.
Everything is better with a beer.