It's been a long time since I have played with a camera but from what I remember doesn't a camera try to produce a correctly exposed image to 18% grey.. so I had a little google and came up with this...
First, white balance will remove any color cast. It does not correct for underexposure. You can use a white or gray target to set the custom WB, as long as it is a neutral color (ie, equal parts of R,G,B so no cast whatsoever.) If shooting flash, yes, use flash to set custom WB. Instruction book says to have card completely cover center ring in viewfinder. I try to get even more coverage. It needs to be properly exposed. If gray card, histogram should be vertical group in center. If white card, vertical grouping toward the right. In no event should the histogram grouping be vertical in the left half.
I don't know how many people on here use histograms to check their exposure but
HERE is a good place to learn about them...
I think it's very good to learn about custom white balances as you can see from
THIS link it makes hell of a difference....