It's preferable to get some scales with adjustable legs so that you can make them all completely level. Easy way of levelling them is to lay some clear hose on the ground, put some water in it, lift up each end of the hose and adjust the scales so the water line is the same on all four. Then just drive the car onto the scales with some ramps of some kind and get someone to read all four readings while you are still in the drivers seat.