Partial Mashing

I was thinking of trying a partial mash (with a grain bag) for a simple wheat beer. All I have is a 3 gallon brew pot. If I figured right, I could only mash around 5 lbs. of grain. Is that correct? Or is that too much?

Also, if the recipe calls for 6 lbs. of DME and I mash 2.6 lbs. of wheat malt, 2.1 lbs. of 2-row and .25 lbs of munich malt that would allow me (at 65% efficiency) to reduce the amount of DME by 2.5 lbs, right?

Before I do this, though, I am figuring out my water situation.

Sorry, I could answer that but after a bottle of bubbly and a good few Son of a Bastard Blonde Ales I'll have to leave it for someone else.