Depends if you're primarily going to be using it as a quad or thruster. I'd jam it right back so your foot is over the rear fin plug. If you're running a quad you'd probably shuffle forwards a touch so your foot is over the quad rears.
I'd suggest wax first to work out where you place your back foot. Youll see that spot from wax wear. I tend to turn with my foot over the two forward fins, so place the year of my pad over the middle of my rear fin. Seems to work.
One thing I hate about demo boards...they dont usually come with tail pads. I have done the splits several times on demo boards...I just dont know how people used to surf without them. All my boards regardless of size or purpose have tail pads....
Completely agree with PRJ.
Back foot positioning is so important on the Dom. At 6ft 2 (1m89cm) but riding a 510 I dont feel this problem particularly as I have to surf it right off the tail due to my wider stance relative to the length of the board. However, if you were shorter relative to your board length then I would say that it was even more important to help get you over the fins.