I think you mean the axial play of the rimm on the axle against part no: 345260-O?
This has nothing to do with offset.
I use Matrix rimms and then you need to cut part 345260-O for 0,5mm to fit the rimm perfectley.
If you want to use the contact tires you can use part 345260-O of a Serpent car, i know that there is more material there and there is no play with the contact rimms.
If X-ray makes part 345260-O same as the serpent one, than almost every RX8 owner need to cut this part. I think because of this Xray choose to take the middle of margins.
![Wink ;-)](./images/smilies/icon_wink.gif)
About the off-set of the rimms. Almost every producer makes his rimms on Mugen off set (or Kyosho because its the same) this has become industry standards. There are small differences between the manufacturers +/- 0.5 - 1mm in offset but this has to do with different molds, materials and production process.