Although there are some guideline sizes for bed quilts, they can vary widely:
How long a drop do you want? Just cover the bed? down to a dust ruffle (add at least 14" to 3 sides) to the floor (add at least 20" to three sides), pillow tuck? (add at least 24" to top). If you can give us an idea of what you are aiming for, we can give you a good idea. BTW, I was a math major. It always doesn't help!