Found this inelegant solution. Any better ones?
Construct 2 end points at the ends of the cylinder
Construct a line from the plane to one end point
Intersect the line and the cylinder to make two points on the edge of the cylinder.
Measure from the plane to the points.........
Construct 2 end points at the ends of the cylinder
Construct a line from the plane to one end point
Intersect the line and the cylinder to make two points on the edge of the cylinder.
Measure from the plane to the points.........