yep i know about snapping. The referenec lines are for the bay window i'm making for the house above. it's just going to be one trapezium extruded down the height of the house.

and i need loads of lines across it for windows, window sills, and all the carvings and cornises to line up. but i'm drawing from a picture in teh image plane and i need to get it as accurate as possible (hence i can't use snapping, i need to be able to drag it).