I've been trying to sell this idea here lately. I get a commission.
Are you on metered water? Right before the fill (coordinate this with the PB):
Start with all the water out of the pool (that's how the plasterer's should leave it). Have everybody in your household take their showers, use the toilets, wash the dishes, etc. Then go read your city's water meter and record the number. Start the fill and resist using any water (or if you have to use the toilet(s), keep track of the number of flushes). No showers. No laundry. No watering the lawns, etc. When the fill is finished, go read and record the meter reading (then go to the bathroom)!!
Calculate the difference between the two numbers, subtract a gallon or two for every flush you snuck in there, and voila! You'll have a very accurate number for pool water volume, which will help you with all your future pool water chemistry adjustments.
Some meters are in gallons, others are in units. You'll need to know which and do the proper math...
BTW, we've got another poster here whose PB dumped the responsibility of the fill into his lap. He's got big problems now because he needed to stop his fill for a period of time.
Do not stop the fill of your new plaster/pebble pool! You want to fill it as fast as you can, in one continuous shot, to avoid getting any kind of ring.
Are you going to be taking care of the pool yourself? Do you have your test kit yet? Have you been practicing with it? Now's the time...