Isn't the width spilt by half onto each side of the wheel? Because by your math the width is only going to the outside of wheel. That's why I'd say for every inch you'd knock off about 6mm...

If a wheel is 10 inches wide with +/-0 offset, it'll have 5 inches on each side. If a wheel is 11 inches wide with zero offset, it'll have 5.5 on each side, not 6 on one 5 on the other...

 

I think you are correct. The rears would stick out around 41 mm more than the stock. It is impossible to accomplish since in most cases if you add 20mm spacers it is pretty much the max. If you'd like to maintain any good drivability and don't want to totally slam the car with almost zero clearance then don't go for Nismo wheels.
By the way Adrians red car is fantastic! How did he do the hood, what paint over the carbon ?

 

The rears at 10.5 +23 will be too aggressive

If you want to listen to those running 255 stretched tires with at least neg 3 camber, prepare yourself to buy rear tires every 5 miles with the risk of damage on those beautifull NISMO wheels

If you plan on doing the hella flush look, I think your better off getting a 3 pc wheel. Having rays NISMO wheels with the stretch look just dosent make sense to me. Actually, having any stretched setup on a daily driven car just dosent make sense at all. At least not to me!

 

Looks tight, why did you say it was a mistake?