39 x 19 = 741 sft

considering the top 3.5' of the pool, the volume would be 741 x 3.5 x 7.46 gal/cft = 19,347 gal

Generally, the average depth of the hopper and slope area is about 1/2 of the difference between the shallow depth and the maximum depth. Generally, the break is about 1/2 of the way toward the deep end (2/5 on shorter pools with a board). So:

19 x 19.5 x 0.5(7.5 - 3.5) x 7.46 = 5,527

5,527 + 19,347 = 24,874 or ~25,000 gallons. PF = 4.8