Usually it is about 1 1/2 inch above the chine batten top. Some have placed them higher and there is a small variance between many boats. Sanderson and I use a cut off scrap of handrail to measure and level the rub rail. Stand back and take a look as that is how you will be seeing the boat on the trailer and in the water. When it feels good... go for it.
I forgot to mention that some even elect to leave them off. It is a traditional piece that might help a little but doesn't have to be there.
While they look nice, they are not without problems. I don;t want to talk you out of anything, but here's some of the reality about rock guards...
On my recent boat which has only this summers' trips on it, I already have some water damage. Those rock guards expose alot of their endgrain to water, and you absolutely need to seal the heck out of them. Mine already have black streaks where water got in, mostly around the screws and lower edge. These were meranti, epoxied and varnished. I really don't care, but what I do care about is that those screw holes to the frames now expose an inlet for water.
If you oil your boat inside and out, it might not be an issue.
If I had a do-over on this boat, I would leave them off. or totally oil the rock guards. Even though they are sacrificial and add nothing to the boats function, I'd still like them to look nice.