Stuck between a 38mm and 45 mm offset for rear rim
 

I have done a lot of searching and reading on the beaten to death topic of rim and wheel selection on the FD. I have been through Flyweight's guide to maximum wheel selection multiple times, as well as downloaded and used the excel sheet that was posted along with it in the thread. Using the spread sheet, for a 17''x 10'' rim in the rear 45mm offset is the option that leaves me with the .27" of clearance between the outer face of the rim and the fender. Problem i am having is, a 38mm offset is suppose to them move the rim more inward toward the center line of the car, but in this case it doesn't seem to be doing so. Not sure if it is the spread sheet I am using, or if I simply don't completely understand what the offsets are doing on the specific rims I am using.

I have decided on the RPF1's as a rim selection and am trying to fit 17"x 9" with a 255/40 or 35 on the front and 17"x 10" 275/40 or 35 on the rear.
Being that I don't want to screw this up and order the wrong offsets, only to find out that they will not work, I was hoping someone here would have done this same setup already and could help sort out my ignorance on the topic.

Perhaps some pictures of the rims with the different offsets, whatever.

Any and all help and criticism is welcome.

Lower offset will move the wheel more outward of the car. The 38mm off set means the wheel is closer to the fender compare to the 45mm.

https://cimg7.ibsrv.net/gimg/www.rx7...db7f02820b.png

So this is accurate then a far as how the offset will move the rim outward. Problem being that the 38 mm offset appears to not allow enough clearance for the rim in the fender. But everywhere I read, even in Flyweights thread about maximum wheel sizes, he recommends the 38mm offset on an RPF1 in the rear on a 10'' wide rim.
Thus I am confused.

Check out the wheel picture thread, has answer to all your questions. FWIW, I’m running the exact wheel size you posted, except with 245/40 and 265/40 tires, mild roll up front and no issue on the rear.

A 17x9.5+15 Is flush on a stock rear fender with no roll or pull. In a 10j the equivalent is a +20. In this case you would want the +35. It will be 15mm more sunken in but depending on your ride height, it will get you where you want to be with a 5mm spacer. Or no spacer at all depending on your preference

17x9 +35 on the rear, with 255/40's. They actually came off of the FC I drove before I bought the 3rd gen.. The rears were pretty tight with the fender on long sweepers, but never rubbed. Fronts were 17x8 with +35. Check the spoke design. The 8's have much more room for bigger brake calipers.

https://www.mazdas247.com/members/Natey/RX-7/FOS_3.jpg

I run a set of RPF1's in 17x10 +38 on all four corners and only had to roll the front. This is the best pic I have of fitment unfortunately.
https://cimg7.ibsrv.net/gimg/www.rx7...fc83954e6b.jpg

Offset fits the tire.

+38 offset will fit 265mm wide tire with minimal camber and no fender roll.

To fit a 275mm wide on +38offset you will either need to roll the fender lip or run 1 deg more negative camber ( so, around -2.5 deg).

+45 offset would fit the 275mm wide tire with no work, but as you know the rim wont look as flush.