Does this calculation on the Pool Calc work?

[Nightmare]

I'm not sure how to calculate the volume of my pool. I've tried a couple methods and numbers range from 20-30,000.

I brought the pool up to a shock level this week and these are the numbers:

FC before shock - 3.0
Added a full 3 gallons of 12.5% cholrine (HASA brand)
Pool running from about 5 pm until midnight
FC at midnight was 20.5

If I use the Pool Calculator to back into the number is comes up with 22,000 Gallons for the pool size to get this result.

The generic calculator on the Pool Calculator is 26,700 Gallons (20' x 40' x 5' Oval). The actual pool is irregular shaped.

I may try to cut the pool into 2 smaller circles and see what number that gives me...

 

[Bama Rambler]

If many chem additions work it out to be 22,000 gallons, that's what I'd go with. Don't base the volume on a calculation or one chem addition.