success (and ye E was made by rotating AC by 90deg in top view so I see its uphill a bit !
Am I right in saying that the viewport that you want the resulting Cplane to be almost same as the existing Cplane view is the one you select the Cplane change option in ?
e.g. If I had been looking at top of a pitched roof house in top view, then wanted the one pitched slope as cplane, I would have made that 3point choice in top view.
I selected the 3point option in the FRONT viewport, but then clicking ‘A’ it was grabbing the rectangle that sits out front of the fuselage. The only way to select ‘A’ was to select it in Perspective view, fearing that this would mess up the fact that I wanted the ABCD rectangle facing me in Front view, and feeling that all picks should be done now in FRONT view after making that option in that view, I went ahead anyway to see what happened, and it worked.
So if one selects a CPlane creation option in an ortho view, one can make the mouse clicks on the curves and points in perspective WITHOUT messing up the command.
Now if I had made that 3point choice in Perspective view, as I very often do, for in that view is where I best see the curves I need to select, then it never works out as expected.
At last, this is the breakthrough, knowing where to make the CPlane option and then which viewports one can make the selections in.
Venturing further, is one ok to make some of the selections in different viewports or must all then be made in just one ?