Before doing the cut on the interior surfaces (9th image above) use EDIT/Align to move the piece to the ground plane:
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(You might use EDIT/Transform to move/rotate it so the base grid intersects the object where you want the measurement's start plane)
Now select the interior groups as in image 9(above) and do SELECT/Edit/PlaneCut.
Make sure that snapping is on (the small S circle needs to be green - if not: Click it) , also the A circle (absolute coordinates) and the W circle (World frame).
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Now dragging an arrow you'll see snapping increments. If needed, refine the increments using the up/down arrows on your keyboard.
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Gunter Weber
Triangle Artisan