If a chair wobbles badly, one of the two logs on which it wobbles needs shortening.
To find out which should be dealt with, cut four blocks out of the same piece of board, and stand the chair on them. Remove the block from under one of the longer legs, and put a weight near the other to keep it down on its block. Note how level the chair now is. Repeat the process with the other leg. Having decided on which three legs the chair is most level, scribe a mark round the fourth, using the block as gauge, and cut off to the mark. Wobbly tables are treated in the same way.