OK, the actual formula is:
(1,000,000/pool volume) * concentration * amount = FC added.
Volume is in gallons
Concentration is as a decimal, so it's .0825
Amount is in gallons as well so 121 ounces is .945 gallons.
So your calculation is: (1,000,000/17,200) * .0825 * .945 = 4.533, or 4.5 is the FC added to your pool from 121 ounces of 8.25%

The rule of thumb makes this easier but this is the formula it comes from. BTW, FYI: as long as your units in pool volume and amount are the same (gallons vs gallons, liters vs liters, ounces vs ounces (!) ) the formula works.