Need help with placement of wither relief pad

I’ve never had a horse with big withers until now.:smiley: I’d like to do some trail riding so I’ll be riding in my western saddle. My saddle fits my horse pretty good except I had to go with a pad with wither relief and the pad is slightly built up. I went with the Impact Gel 30x33.

Using a wither relief pad is new to me and I need some help on proper placement. Do I place the opening of the pad directly on top of the withers? When I do that the pad seems too far forward and barely anything showing at the back of the saddle. If I slide the pad back slightly and the opening is more towards the back of the withers it looks more evenly distributed under the saddle and the built up is portion is where it needs to be. I want to be sure my horse is comfortable and the pad is not pushing down on his withers.

TIA