How can proper degree of medial rotation for an AP oblique with mortise rotation be determined?

the leg and foot are rotated until the intermalleolar line is perpendicular to the image receptor
the leg and foot are medially rotated exactly 20 degrees
the leg and foot are rotated until the intermalleolar line is parallel to the image receptor
the malleoli are perpendicular to the image receptor

the leg and foot are rotated until the intermalleolar line is parallel to the image receptor

Rationale: The mortise oblique requires the foot to remain in a natural position. The leg and foot are medially rotated until the intermalleolar line is parallel to the image receptor, which could be approximately 15-20 degrees.