Instead of cardboard you might want to invest in a few small washers (the nuts and bolts kind of washers). They should be pretty uniform in size and allow you to make consistent adjustments. Once you have figured out how many washers you need at each corner then it might even be possible to to unscrew the legs and add the washers between the legs and the base and avoid the possibility of having someone mistake your bits of cardboards for trash and throwing them away.