I can only tell you how I do it........I measure the length of the quilt........add just like you said...seam allowances. If you buy fabric that is 45 inches wide at least, then you have plenty for the top and bottom also. Hope that didn't confuse you. Yes I know a lot of people are going to say you don't need that much fabric.......however, if you buy fabric that you will use in something else.....it is not a waste at all. Not to mention.....it's simple. Always sew your sides on first then the top and bottom last.
Hope that helps.