The math gets you a multiple of 9 with 81 being the highest possible.
Before you hit the big blue button, check out what symbol is adjacent to 81, 72, 63, 54, 45, 36, 27, 18.... it will always be the same symbol, but changing each time you push the blue button.
Only the multiples of 9 are important, the rest are just the magician's misdirection ploy.