If it were me, I'd use Michaels Bleach Calc once again. Measure the length, come up with an average width and depth. Input those under Oval maybe in the More Calcs, Pool volume tab. You can probably come pretty close. I kept doubting that my pool was only 16K gallons at the beginning of last season so I tested FC (.5 with no CC), added 1 gallon of 6% bleach after sun down and tested again 1 hour later to find that FC was now 4. By backing into the bleach calc, I determined that 16K gallons was probably right. This wasn't very scientific but did reinforce what the builder had told me was his best guess.