For my underpadding, I also added a layer of 5mm underpad at the back of the tray to help make a very slight ramp up the back. I forgot all about it, but you can see where it starts part way into the heel of the cutouts for the footholds. I put it in between the two layers of 8mm.... so it was a full layer of 8mm, the 5mm kick tail at the back, and then a layer of 8mm with the cutouts for the feet. I'm quite pleased with the results, although the 5mm kick tale isn't all that noticeable.
A lot of guys really like the single layer of 25mm, but that seemed a bit much for me. That's a whole inch of underpad without the actual turf on it. I definitely wouldn't have enough room to put the 25 into my WCF footholds because I wear wake skate shoes when I ride.
If you do two layers of 8mm or the single 25mm layer, make sure you cut out the area around the stern eye a little. Most of the eyes don't have a long enough section of threads to go through all that turf, underpadding and the rear of the ski. As you can see in my picture, I only had the single layer of 8mm padding around the eye.