Generally, you can plan on 1 pound grain = .75 pounds LME = .6 pounds DME.
Since wheat DME is about 55% barley, 35% wheat, you can just replace it. If you can mash about 5 pounds of grain, I'd do something like:
2 pounds 2-row
2 pounds wheat malt
1 pound munich malt
1/2 pound crystal
4 pounds wheat DME (actually calcuates out to about 3.6 pounds, but I'd go with an even 4 pounds, to make up for possible poor efficiency on the PM)
Don't forget to adjust your hopping rates, if you're doing a less than full boil!