I've drawn the pivot line on the plans. It is designed into that stab pivoting hardware to be perpendicular to the joint the stab makes to the fuselage.

You can think of it as if it were a hinge line. If the stab was actually a stab/elevator with a hinge line, it would be along the heavy green line I've added to the plans. And as shown, it passes at the .24C MAC point.

I did a quick proportion check of the plans and the point on the MAC the pivot line runs through shows that point to be 30/125 or 24%.

The main problem with the design is that the pivot axis is actually not in line with the actual rotation of the stabs that are being held and rotated in a different plane.

You are correct to dislike the design. It shows little understanding of the geometry of the tail. And is quite a lot of work to do, that could be done more easily in a number of ways.