I've never done any WP but I've always heard that your body should be in a position that if the horse was slipped out from under you, you would be standing upright. I try to ride this way, not sure if its correct or not. I do pay attention to "heel to ear" position even though I'm just trail-riding.
I found this description on stirrup position for WP. Thought you might find it useful.
Legs and Feet: Stirrup length is important. When standing up in the stirrups with your heels lower than your toes and your knees slightly bent, there should be a space about 2 1/2 to 3 inches (a hand’s width) between the buttocks and the saddle. The knees and thighs are rolled in so that the flat part of the knee is against the saddle. There is light contact with the horse on the inside of the calves. Ankles are flexed so the heels are lower than the toes. Only the balls of the feet are in the stirrups.
Here is the entire article if your interested-
Western Seat Basics – America’s Horse Daily
ETA- I did get butt craps when I first started trying to ride like this so be prepared