I'm creating a model of a vertebra and I know the horizontal distance between the ends of two extensions which do not share a common point.  These extensions are about 30 degrees off from the right plane, pointing different ways.  They are not coplanar from the top.

Basically I want to set the value dx as would be measured using the Measure tool in a sketch that is not in the x-plane.

Is there any way to do this?

Thanks.