Actually, now I'm more confused than ever. I found this on rims at Club Lexus.com... http://www.clublexus.com/forums/showthread.php?t=158592
Offsets: The stock offset is +50mm. When stock wheels are mounted to our cars, they appear to be “sunken” (tucked into the fender). Too low of an offset will make a wheel stick out, passed the fender.Many people choose to go with a lower offset, or run spacers, so the wheels will not appear to be sunken, and come to the edge of the fender, for a more aggressive/clean look.
you can go as low as +38 on a 10” wide wheel in the rear without any fender modifications and as low as +22 on a 10” wide wheel with your fenders rolled. your tire size plays a big role here.
From this it seems that positive 40mm will be fine, so, considering what you said, I am now more confused.