Here's how I would do it.
First borrow a big mirror from your wife.
Put the Prather gauge in front of the mirror.
While looking at the image of the gauge in the mirror, put on the pusher prop.
On a serious note
, why repitch? There is a lot of flexibility in the electric system to retune the rpm to match the flying characteristics. So if my 12-6 was too fast, I would just dial down the rpm, and if too slow, dial it up---within reason of course.