Under the modern rules for American Pool the table is not marked with spots as the cue ball can be played anywhere on the table.  However, if you wish to mark the table with a black marker pen for the English game, proceed as follows:  The black ball, which is set up centrally in position one down from the top of the triangle, should be moved into position by rolling the triangle of balls, so that, on each side, the black ball is in line with the bottom corner ball of the triangle and the corner pocket.  You will find that this is just slightly nearer the end cushion than central to that half of the table.  The black ball can then be removed and the table marked at this point on the table.  The baulk spot  (and line if required) can be marked in the same position at the other end of the table.