Bottle priming calculators assume you have X amount of residual co2, but they don't factor extended storage time. Since you made this in October, you probably have very little co2 in solution. So if you want to carb to a specific level, you will need more sugar than the calculator specifies.
Adding yeast may not always be necessary, but it's cheap insurance for carbonation. A longer storage situation would be a good time to use some bottling yeast. Champagne yeast is less than a dollar and it's a good choice for this purpose.