IR index mark

Fellow Apuggers,
I've got an Arax 85mm tilt/shift lens. Trouble is, it doesn't have an IR index mark on the focus scale. Is there an easy(ish) way of knowing how much focus shift will be needed? Many thanks, Blights.

Maybe not "easy" but I would do the following:

Focus on a distant landscape (infinity) with the lens wide open.
Bracket frames with the infinity marker on the lens lined up with the middle line, then lined up with each aperture mark on the depth of field scale. ie 4, 5.6, 8, 11. Then examine the negatives for the sharpest one.