Wheels for Fd- I am measuring something wrong
 

I have an FD with Avanti 18x8jj 225/40/18 wheels offset of ET36. I measured the wheels with a ruler to confirm and the mounting face fo the wheel is infact 36mm from the centre. The wheels just miss the guard but I have about 50mm clearance on the inside (between the tire and the suspension). I wish to fit wider wheels so I went to the post

Maximum Wheel and Tire Width Fitment Guide for the FD

https://www.rx7club.com/suspension-wheels-tires-brakes-20/maximum-wheel-tire-width-fitment-guide-fd-892901/

and read that it is possible to fit 9.5 wide wheels and the ideal offset for these wheels is 48 front and 40 rear.

SO far so good.

But when I went to a wheel offset calculator like

http://www.1010tires.com/WheelOffsetCalculator.asp

the 9.5 wheels will be 7mm (48 offset) and 15mm (40 offset) closer to the guards. So they will infact rub against the guard as my present ones are almost touching.

So what am I doing wrong?