It'll help to knit a swatch. Cast on about 22-24 sts and knit it the pattern that you're thinking of using for the blanket. When you've knit abot 4-5inches, you can measure the number of sts that you get over the center 4inches. Then you can use that and the width that you'd like for your blanket to figure out how many sts to cast on. (Xsts/inch times finished width in inches in blanket = the number of sts to cast on.)
salmonmac is right and that is what I'd do. That is how it should work, but in reality I find that, for me at least, swatches lie. But it would be give a good ball park idea, but probably not be perfect.