You place the inuslators anywhere on the post from top to bottom. The posts need to be in the ground a foot or 2 depending if it is a thin rod or heavy T post.
So, you need a post that is longer that the height you want to go. You don't need to buy a post exactly as high as you want the fence?
For my cows, I am almost 6' tall & I place the single wire _just_ so I can step over it without harming myself. Seems the best. Placing it too high causes more problems with them getting under it. Placing it too low once in a long while they will go over, but rarely.
That's cows, I don't know much about horses.
Edit - Ok, rereading your message, I see you are temp or cross fencing - I got sidetracked with some other messages.

The 4' step in posts should work for average horses of average temperment. If you are more than 3.5' high with your single wire, I would think you have a lot of trouble with them going under. Likely you will end up with the tape 2.5 - 3 feet off the ground at the most.
--->Paul