I would do it by a workflow similar to:
start-situation
cplane
set the cplane to 3 points, and move the 4th point away randomly
subDPlane
draw a _subDPlane with 1x1
if you need a parallelogram additionally use _shear
stitch
that s the reason why I moved one point at the beginning:
now the selection for _stitch is much easier.
make it worth before stitch
… is a super nice workflow to more easily select the edges / vertices…(and overcome the limitations of this command regarding selection)
analyse
if you really need to measure wether they are parallel:
_angle will allow you to measure with 4 points.
(@theoutside it s a pity or even a bug, that _angle does not accept 2 SubD edges)
… but I think, if it is not parallel, you have to fix it - so above will make sure you have a cap and parallel edges.