Always keep in mind, the fixation has 2 major responsibilities, when it is applied. The first, is to create a "shelf" for the ink to sit on. It is the overall barrier that prevents the ink from soaking into the fabric. The second purpose is to cause the ink to "Gel". The gel process not only allows each layer to rest on top of each other, but also forces the ink to hold its place on the garment.
With white garments, since you are to layering the ink, there is far less of a need for more fixation. You are just aiming to create the "shelf" for the ink. With dark garments, the ink is layered, and also needs the fixation to penetrate through each layer, for the gel process to occur. This is why dark garments need high percentages of fixation, and light garments need much less. Given the explanation above, this is why the percentage of fixation can vary so much, from garment to garment, brand to brand.
I would suggest starting with about 10% spray, for white, then work your way up from there...if needed.