Ecory,
To lock the inferencing (temporarily) to ANY axis hold down the SHIFT key on your keyboard.
Very useful.
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This way you can lock to the R,G or B axes.
Similarly, as in your case, when you see the Magenta coloured axis light up
(= perpendicular to edge or parallel to edge) - hold down the Shift Key also.
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Holding down the Shift key will also lock the inferencing (temporarily) to any arbritrary axis (Black Line).
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As mentioned, guidelines can also be used.
You could also draw a temporary line leading away from you ramp as a guide which you can delete afterwards.
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Hope this helps
Howard L'