The easiest way would be to use 3d dimension, that, at the moment, is not yet implemented in Rhino 6, but hopefully will be included in Rhino 7 (or a future update of Rhino 6). I imagine it could work this way:
- Pick the start point for the desired distance.
- Pick the end point.
- Snap to some object that closely matches the plane at which you want the dimension to be aligned. It should work exactly like the last click of the “Rotate 3d” command where you pick the angle of rotation while the start and end points work as an axle. Or hold down Shift (if Ortho is not activated) to align to the World axes (or current custom CPlane axes) depending on the ortho angle setting (usually 90 degrees).
It’s so simple and I wish it had been already implemented in Rhino…
