It doesn't matter if you are doing extreme highlights, zenithal, OSL, or any other highlights, you have to make sure that you understand how and where light will hit the model and reflect, and this should be your guide to highlighting.
Let's take the Sicaran I am working on. The idea here is to edge highlight the armor plates, using 360 degree top light.I will use line highlights on the larger armor plates, where there are not sharp edges, to pick out details.
I took my chosen highlight color (1 shade up from the blue, so Calgar Blue), and using a fine detail brush, at a 45 degree angle, using the side of the brush, ran it alonng the edges of the armor plates, resulting in a very fine line.
I take my base color, and trim it up, and then the highlighting is done. Of course, I could go a step further, taking an even lighter blue (like PP Frostbite) and applying it to just the highest edges.
However, the point is, I have chosen my light source to be at a rough 45 degree angle and all around the tank.
The highlights hit all the top edges, but only the vertical edges on the sides. If the edge is pointed downward, no highlight.
On this part, you can see the highlights across the less sharp edges in the armor plates.