I have this same problem almost every year. I usually print out the drawing that Boltman posted. I use a calipers to measure one of the dimensioned features and then calculate the scale of the printout in inches per inch. For example if the width of the field is 319 inches per the drawing and its 7.1 inches on the printout then I know that every inch on the printout is equivalent to 44.9 inches in real life.
Low tech for sure but it works for me.
P.S. Why yes; I do have a 12 inch dial calipers.
