I used 63 because it's easy to AND against, and uses the least code.
Since it's binary 00111111, when you AND against it and the result equals 0 you have either 0, 64, 128, 192.
At least right now, the timer starts at 0, then subtracts 1 to get 255, then subtracts until it reaches 0 again. You could compare the results of your AND to the number 60 but any time the binary representation of 60 becomes true, whether the number is a multiple of 60 or not, the countdown would tick.
In this case, though, since I'm already ANDing 63 with gameTimer it probably would only really be those extra couple bytes and 2 CPU cycles, but 63 is close enough to 60 that unless you're speedrunning your average player's not going to object to 0.05sec drift.