Here is a link with the offsets for the various factory wheels on the i8:
https://bimmercat.com/bmw/EN/search/...i8/USA/36_2467
The size of the wheels (21") is not really the defining dimension here. What mostly matters is the WIDTH of the wheels.
Looks like they are using 36mm for 7" wide, 40mm for 7.5" wide, and 50mm for 8" wide wheels.
Generally, if you are trying to keep the wheels centered in the wheel well, you would simply add 12.7mm offset (25.4mm/inch / 2) for every 1 inch width increase of wheel. At some point, you are going to hit something (suspension inside or fender outside) and then you'll have to move the wheel one way or the other to maintain clearance.