Have you ever knit a project, say a blanket or scarf, that was the same at either end? If you're like me, then you probably push your yardage to the limit, knitting as far as possible before starting the finish edge. To avoid the frustrating situation of having worked too far and having to tear back because you ran out of yarn, or not working far enough and being left with a minuscule amount of yarn to be stuffed into a jar and all but forgotten, consider using a scale [this is the one I use].
To ensure that you get the most out of your yarn, weigh your ball before you cast on and after you've completed the beginning border. When you find the difference, you'll know exactly how much yarn you used, and exactly how much yarn you'll need for the finish edge.