you could select your axis then top plane to parralel to, or plane 5 perpenicular to.
Like Mr. Hutchison stated, select the cylinder axis as your first plane reference.
But I would select the top face of the ringed boss in the upper left of your image for the second plane reference.
The cylinder is square to this geometry and I think it's best to use this surface.
Select Axis of the cylinder as your first ref. and Plane 5 as your second ref. but in stead of coincident relation select Perpendicular. I hope that helps.