Re: avoiding guesswork, make sure you don't have "floating pixels" that is, pixels that arrange in a square and can be swapped. r16c2 and r17c3 can switch and be r16c3, r17c2.
True diagonal lines are ALWAYS floating, even if they're more than one pixel thick, because you can point them either or /. Connect some but not all of the pixels to create a slight curvature will usually fix things. Also, slight asymmetry can help. Then you can't swap across the vertical median.