All of the following will work and be within the tolerances you need on an XI
225/35/19 front, 255/30/19 rear
225/35/19 front, 265/30/19 rear
235/35/19 front, 265/30/19 rear
235/35/19 front, 275/30/19 rear
The decision now becomes what will fit with YOUR setup. How much you are lowered, what wheel offsets you're running, etc., etc.