I'm no expert since I'm here asking questions too.
Mine is on top of a 6 inch wide piece of wire mesh, it's narrow enough from front to back that I can tuck the heating pad and towel down to the ground (but not on the ground, they can't sit on top of it when they are also under it) in back and it still covers the front edge of the mesh too. I pushed it up against the back of my dog kennel brooder as well as I could, but my chickies discovered that they could push stuff around back there and pop their heads out from behind it! It was really cute. They decided that it wasn't much fun after a while and I still try to keep it pushed up against a wall as best I can so it's more cave-like in there. Mine is at the height of their backs when they are standing. At first I had it too low and even though they could crawl under it they were panting in there so I made the whole thing taller. I hovered over my chicks the first day and it only took a little while for me to notice that! Mine is about the same height all the way back, but since the heating pad comes down to the floor on the sides and the back the chicks can snuggle up against the sides when they are cold. I watched them a lot for the first day since I hatched them from eggs and it was my first time hatching AND my first time with the MHP I was a nervous mommy! But chicks are resilient and they will let you know if they are uncomfortable.
I hope that helps.