PF = pool factor.

If you add 1 lb of chlorine gas to a pool with a PF of 1 (120,000 gallons = 1,000,000 lbs) you get 1 ppm of chlorine.

If you add 1 lb of chlorine gas (100% chlorine) to a pool with a PF of 10, you get 10 ppm of chlorine.

If you add 1 lb of dichlor (55% chlorine) to a pool with a PF of 10, you get a 5.5 ppm of chlorine (1 x 10 x 0.55)