In case somebody else finds this useful: when matching scaled blueprints as background images to Blender units for reference. take the width of 1 base unit (like 1 meter) in pixels from blueprint. Take width of entire blueprint image in pixels (the image you'll use as reference in viewport).

then if you want 1 Blender unit = 1 meter in blueprint image:

(image width/base unit)/2 = size factor in background image pane (View->Background Image...)

As an example: a front view image that is 400 pixels wide in which the 1 meter scale is 47 pixels wide: (400/47)/2 => 4.25 scale factor to give 1 Blender unit per meter.