By putting the front tyre sizes in the Your Existing Setup section and the rear tyre sizes in the Your New Setup section. All being equal, a staggered setup of 225/45r18 8j front and 255/40r18 8.5j rear will give a rolling circumference difference of 0.23% - well within the 2% tolerance.
The closer to 'zero' difference between axles the better for xDrive. Less wear on the drivetrain and/or less tyre scuffing. We have to also remember there are tyre tolerances, they are quite wide for generic sizes. RC can be nominal +1.5% / -2.5%, according to DIN standards. Peter
Yes of course, which is why I said all being equal. Same tyre and sticking to one manufacturer. Obviously having 4 different tyres - even though they may be the same size - from 4 different manufacturers would be a bad idea. Best would be * marked.
