If you start out with a full bobbin in the bobbin case and a full bobbin on the spindle in place of a spool of thread they will not run out at the same time because the top one will use more thread as it loops around the bobbin thread. It certainly gives you a closer approximation than just using a spool of thread but there is a difference. I often use a bobbin in place of a spool of thread if I want to use it up. When I'm piecing I'm not really fussy as to what color I use since it won't show anyway. The more money I save by being thrifty with thread the more I have to spend on fabric!!