The best way that I know of is to run it. Your boat will sit low in the water at slow speeds then lift as you speed up. If the prop cavitates once on plane or in the corners, it's too high.
When I saw the TC 31 for the first time, it was still in the box at a LHS. To me, it appeared that Pro Boat had the strut raised to make it easier to package for shipment.
A starting point would be to have the centerline of the bottom of the strut or centerline of the flex cable about even with the bottom of the sponsons then you can raise it just a very little at a time until you run into the above problems then lower it down a touch.