There it is, proof that my crackpot calculations work. If you are irritated by particulars of the new MetroCard pricing scheme, you can still buy a card with an integral number of rides. But make sure you use plastic or have exact change, lest you end up with a handful of golden dollar coins featuring Paul Giamatti's John Adams' bug-eyed visage.
I like the vaguely mystical overtone that you have to know the secret code and apply it in exactly the right way in order to receive the magic number 18, for life. (חי)
In not entirely unrelated news, if you don't visit New York City often enough, or worse, don't ride the subway often enough, you might occasionally find that one of your MetroCards has expired while still holding value. Horrors.
If your card is less than a year expired, you can trade it in for a new one using one of the handy machines around the stations. If your card is older than a year, you'll have to get one of these claim forms and mail in your ancient card. Ask at any friendly former token booth.