Ah, good old conic sections! There would be a mathematical way to figure this out but it would make my brain hurt. A truly accurate mask for the white section would take on a C-shape but I would go about it differently.
First of all, check that the blue portion behind the prop corresponds to the seam where the spinner and backplate meet. If so, that's half the battle - just paint the blackplate before gluing it to the spinner.
The problem with cones is that if you use straight pieces of tape, the edge of the tape on the smaller diameter will pucker and you won't get a clean edge. The trick is therefore to use a very thin piece of tape to define the edge you want to mask and use progressively wider pieces away from the edge where all you need to do is cover over a painted area. Assuming that you can define the backplate edge per above, I would proceed with painting the spinner in the blue colour first (normally you want to paint light colours first but in this case, it won't work). Once your blue has dried, place the spinner pointy side up on a dead flat surface and rig up a sharp pencil horizontally at the right height against the spinner to define where the white edge should be. Turn the spinner against the pencil, keeping it on the flat surface until you get a good pencil line around the spinner. Now, with a very thin strip of Tamiya tape, carefully apply it along the pencil line all the way around the spinner. Once you have a nicely defined line, continue masking the remaining blue areas or use liquid mask. Then spray the white a few very light coats at a time until you have a good, solid colour. Repeat this step at the rear white/blue demarcation line if it's not on the backplate seam, this time masking the white edge and painting the blue.
If you're careful and patient, you can get a decent ring on the spinner. I used this technique to do this rather complicated scheme: