Measure

You can use (or make) a coordinate system to use as your reference plane and uncheck the "use as axis" box so it give projected distances. (my preferred method)

OR

You can measure surface to surface (use as axis checked) and manually subtract out the 2 radii (can be quicker than making a coordinate system if the angles are really odd)

Measure between axis and subtract radius of each.

Probably the most accurate, depending on orientation, draft, etc, would be to make a plane between center axis and create a sketch.  The issue is you have to define the measurement want to CREO.