Totally feel you on the "just guessing where the lines go" part. I once spent a whole weekend trying to trace a wire based on pure hope and a bad drawing on a napkin. Let's just say my success rate was firmly in the "accidental fire hazard" category. It’s amazing how much time you can waste pretending you know what you’re doing up there.
Cheap toners often fail when you need them most, giving weird signals. That guessing game patricia mentioned just gets more confusing with a bad tool. @the_kai's weekend of hope and napkins is a perfect example of why that happens. A decent toner actually tells you where the wire goes without the drama. Skip the dollar store version unless you enjoy redoing your work.