Your lace pattern should give you a number that tells you how many stitches your lace uses–usually a multiple of X plus Y. For example, if your multiple was 10 plus 3, you’d cast on the width you want using any multiple of 10 plus 3 more–13, 23, 33, 43, etc.
Use the needles called for on the yarn label and do a swatch of the lace pattern. That will give you an idea of how many repeats you’ll need to get the width you want.
As for how much to buy? I’m afraid I can’t really help you on that one. :??