Amazing oasis you have there!
You can track your pool chem additions and see if you're usually over-shooting or under-shooting the expected change, and then adjust your volume accordingly. PoolMath provides accurate calculations, so this method works well to fine-tune pool volume. You could start in PoolMath with your best guess and go from there.
Pool volume inaccuracy will only cause significant problems with respect to chem additions that can't easily be reversed, such as CYA, CH and salt. Until you're sure of your volume, add 1/2 or 2/3 as much as PoolMath recommends, rather than the full amount. Then you can add the rest if needed, and also gain some information that helps with dialing in the pool volume.