Actually, I'd recommend dumping out 2/3 of that yeast cake before re-pitching on it. Yeast generate esters during the growth phase. In some styles, those esters aren't desirable, but you want them in a wheat beer to generate the banana, clove, or bubblegum flavors.
Pitching directly on an entire yeast cake sidesteps the growth phase, leaving little opportunity to produce those esters.
Better even than just guesstimating would be to wash the yeast (instructions here), and prepare a new starter culture. It's very easy to do.
To figure out how much yeast slurry to use, you can go to the pitching rate calculator at Mr Malty and use the calculator to determine how much slurry you need, assuming that your washed yeast is 20% or so non-yeast solids.
I think you'll have better results in a wheat beer this way.