transform orientation

I am modeling a house and have ran into a problem knowing how to work on an angled wall.
The wall is modeled and I have created the initial opening on one side of the wall.
The question is, how do I move and size it so that it remains “on the wall”?
I want to set exact dimensions from the cursor but don’t know how to set a coordinate system at the cursor and then move relative to it.

Please advise.

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Hide all detail but that wall. Align view to it, and use coordinate system normal. Show distance of edge so that you can accurately drag opening.

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