I understand offsets but for some reason I can't get my head around that diagram
That's because it doesn't reference the edge of the wheel arch which is what you need to see to have an idea of how far inside it the wheel will sit.
The X5 should be very similar to the Range Rover in poor conditions, given that BMW owned range rover at the time the X5 was designed and a lot of the guys from Gaydon got sent sausage-side