Heinz my preference is around a 10mm minimum gap between the axle and rebound rubber to ensure there is enough extension travel for optimum handling and comfort. There is a simple suspension check that might be worth doing particularly if you have or are intending to reduce the rebound rubber’s thickness.
Jack the rear of car supporting it on the frame and let the axle dangle. Now check the axle is nicely sat on the rebound rubbers both sides. You should see some decent compression in the rubbers but defiantly no gap. Two things to look out for. Firstly the u-bolt nuts are not contacting the chassis and the other is that the axle is not being lifted/restrained by the dampers. This is only a crude damper extension check. Remove the top fixing and check the stroke if you want to be certain.
