Looks like you already know what you're doing ~
Yes, you take 1/12 of what you would earn for the year.
So assuming you had $10,000
....
$10,000 * 3.15% / 12
$26.25 monthly
Just remember that you will earn slightly more each month (a few cents, really) because of the compounded interest. You continue to earn more interest on the interest you receive....