I'm crocheting an afghan, but I want to make it wider. How do I the math for this?

When you are unsure of the number of stitches needed to create the pattern. I usually just count how many stitches they say to crochet into and also count those they say to skip. This should give you the number of stitches required to make the pattern stitch. If all else fails, I resort to making the starting chain longer than the completed afghan width, those extra chains can be worked into the project once you have made the stitch in the desired width.