I would try to use following work-around:

• measure on 2 passes the distance between mid-tractor lines and set this as swath width (normally this should be the working width of your plow).
• check there is no offset from hitch attachment
• register my first path as reference line
• start with EZ Steer guidance from there on.

If your plow is set up right, you should have the same distance between every pass. If you have an offset from the way the plow is linked to your tractor (say 1 inch to the Right) this should be the same (1" to the Right) on the return pass and this offset should be entered into EZ Steer settings.

May be your plow has to be adjusted to draw straight? This is a way I could understand that it would request an offset on both sides.

Have you tried measuring your tractor midline distance on 4 folowing passes? 

JMHO