10mm difference is within spec. If it is out of spec you can buy pads for the rear springs which will equal up the rear and bring the front into spec. The pads are in 5, 7.5 and I think 10mm thicknesses. As the suspension is a lever the increase will be greater than the thickness, try a 5mm on the rear. Buy them from BMW not ebay, they only cost about 7 or 8 quid