More precise positioning of Planar Surface tool?

Is there a way to get more precise movement when adjusting the corners of the Planar Surface tool? I’d like to move it at 1/10th the speed is it now. It feels like it’s snapping to whole pixels or a really coarse grid.

Keyboard modifiers? Nudge commands?

Yes, there is a keyboard modifier to slow the positioning of a surface corner: Shift+CMD (or Shift+Ctrl on windows/linux).

Fantastic! Thank you!