(a) In determining the tax on beer removed in kegs, a barrel is regarded as a quantity of not more than 31 gallons. The authorized fractional parts of a barrel are whole barrels, halves, thirds, quarters, sixths, and eighths, and beer may be removed in kegs rated at those capacities. The following keg sizes are also authorized at the stated barrel equivalents:

(b) If any barrel or authorized size keg contains a quantity of beer more than 2 percent in excess of its rated capacity, tax will be determined and paid on the actual quantity of beer (without benefit of any tolerance) contained in the keg.

(c) The quantities of keg beer removed subject to tax will be computed to 5 decimal places. The sum of the quantities computed for any one day will be rounded to 2 decimal places and the tax will be calculated and paid on the rounded sum.