Only your experience has the answer as stitch length and closeness determine how much you might need. I quilted a 48x60 size recently in shell pattern using walking foot and it took 5 bobbins. Second quilt same size and pattern took little over 3 bobbins as thread was thinner. So, I don't believe there is any formulae to determine how much you will need. Also, different machines take different amount on the bobbin. Your tension also effects it. Sorry but I don't believe there is any cut and dried answer. If you need to know for the future, then I believe you will have to buy more and then after finishing to keep a record of the amount used for the next time.