So I assume your wheat comes in the form of DME? That's how I would do it if I were doing partial mash for a porter. Actually, IMO, a Porter is probably the best style for extract brewing, as chocolate malt, crystal malt, and black malt (or my preference, Carafa Special II) all steep well, and the residual sugars left over from the extract isn't so offensive in a porter. If I were doing a partial mash, though, I wouldn't bother with this style, as there are no real "must mash" malts that I put in my porters (though I know many people do put Munich malt in their porters, I do not). And wheat malt can be a PITA to mash with in a tiny system, IMO.