Scribner Decimal C
The Scribner Decimal C is used in the Midwestern and Northern regions of North America. It was published in 1945 in the University of Minnesota and uses the help of a table.
(0.79*D² – 2D – 4) * L/16 = FBM, rounded to the nearest 10 FBM (313 ~ 310, 317 ~ 320 etc.)
D – Small-end diameter in inches, rounding half down if the diameter is exactly .5
L – Length in feet