It depends on a lot of things like shape, size of the diving hopper, slopes, stairs and so on.
Here's a way you can test to find out a still rough estimate, but potentially a bit closer:
Buy chlorinating liquid
- 8.25% bleach from a high volume store where bleach is stored indoors, with a manufacture date around 16140 to 16170
- 3 jugs of 121 oz or 128 oz unscented plain bleach (not splashless)
Run pump(s) on high for 60 minutes leading up to a time in the evening when the sun is off the pool
Test FC using the 25 ml sample size where one drop = 0.2 PPM FC
Add the bleach
Keep the pump running for another hour
Test FC again using the 25 ml sample size
If your pool is 75,000 gallons, the FC rise should be about 3.2 for 121 oz jugs or 3.4 for 128 oz jugs
Or you can track your use of PoolMath, and record predicted vs. actual results for all chemical additions. If you're undershooting and overshooting equally, you have the volume about right. If you're undershooting more of the time, then raise your pool volume estimate in PoolMath, or vice versa.
There's a very accurate way to do it with TA, found here:
A Chemical Way of Calculating Pool Volume