It's best if you can keep the diameter of the tyres the same. Otherwise one set of wheels will be rotating at a higher speed than the others, and the ATTESSA system will think you have wheelspin.
245/35/18 = 628.7 mm
285/35/18 = 656.7 mm
So your rear wheels will always be spinning approx 5% slower than the fronts.
See if you can get 285/30/18 = 628.2 mm. Then both sets of tyres will be the same diameter.
Hope that helps,
Gary.