Brilliant tip...Something that definitely falls into the "Why didn't I think of that" category.
Use a Strip of Tape to Help Hang Pictures Perfectly Every Time [Clever Uses]:
Getting a picture perfectly hung on the wall is already a difficult task, but it's made even harder when the frame requires two screws or nails. To mark the length between the hangers, DIY blog It's Overflowing recommends using a strip of tape so you can measure the distance and drill the holes straight.
Grab a piece of cheap tape (painter's tape or masking tape work fine) and stick the tape between the two holes on a frame. Now you have a good gauge of the distance between the holes. Next, take the tape and stick it on your wall, check to make sure it's straight with a level, and drill your holes. It's an incredibly simple way to ensure your holes are drill right or your nails are in place. Just don't forget our tips for hanging pictures without destroying walls before you start drilling.
Simple DIY: Creating a Window-Pane Mirror | It's Overflowing via Reddit