Thank Michael and Peer.
Michael, I will construct an example file to post. It may take a bit of time, but well worth the time if your attention to the challenge can bring the intuitive ease of MoI 3d to a classic and very daunting task of cutting compound angles. This is usually faced in cutting crown moldings. Rhetorical Question: If the walls are square to the ceiling and to each other, why can't you just set the mitre to 45 and the bevel to 45 and cut a perfectly square crown moulding mitre?
Peer, I appreciate your explanation of how to construct a helper line at the intersection of the two planes. I would be looking for an angle measurement (or a pair of complimentary angles adding up to 180). Is it possible to snap a line segment starting at and perpendicular to a point on the helper line and ending at another point on the surface of one plane, then beginning at the same point on the helper line, snap a line segment also perpendicular to the helper line to another point on the surface of the second plane? I think that the two lines would be coplanar (a third plane) and the angle between the two lines would be the angle of the 1st and 2nd planes relative to each other.
Also is it possible to use DIM angle command to derive the measurement of this angle?
Thank you,
Dan