Most of the knowledgeable breeders that I've talked to think that a white halo is NOT a reliable indicator of a WF split. In fact I only know one person who thinks that it IS a reliable indicator. Birds can have the halo without the split, and can have the split without the halo. I suspect that the presence or absence of a halo is more related to how intense the yellow coloring is on the rest of the bird. A WF split might or might not influence the general intensity of the yellow, but there are other factors that can do it too.
I have an article on split signs and how well they work. Pied is still the only mutation that's considered to have a reliable indicator of a split. http://www.littlefeatheredbuddies.co...ielsplits.html