In my screenshot I rotate the whole object in edit mode (any random angle so the axis won't line up with the grid). I want to add a segment or edge (blue line in the screeshot). I can do this with subdivide, but how to precisely make it exactly 1 unit from the red edge? What is the fastest workflow? is there anyway in Blender I can hava a custom construction plane?
You can select a face, turn the transform orientation to "normal", then click the plus button: this will create a new set of trasform orientation that you can use to manipulate your vertices, edges and faces along the three normal vectors of the first face you selected.
Then in the overlays section you can activate the measurement function, to get infos of edges lenghts and faces areas.