As you've already encountered, knowing your pool's volume comes in handy for many things. Will your fill water be coming from a water source that is metered (like from your city, through a water meter near your curb or street)? Here's a way you can find out what your pool's volume is, very accurately.
Start with the pool as empty of water as possible. None if you can manage it.
Mark where you want your water level to end up with a piece of tape. This is usually half way up the skimmer opening, but other things might determine that as well. The point is, when the water is nearing where you want to stop, it's not always easy to tell where that is, either because of the reflection in the water, or the refraction of the water. A piece of tape is much easier to see.
Have everyone in your household take their showers, use the toilet, wash the dishes or clothes or whatever, before you start your fill. The idea is to use virtually no water during the entire fill. Be sure any automated irrigation system is shut down.
Go out to the street, find your meter, make note of the meter reading and/or take a picture of it.
Start your fill. You can use as many hoses as you want. With 38K to fill, that should take overnight. Start your fill late in the day, after shower time, and then go to dinner or a movie. Family night! Celebrate your new pool. Come home and go to bed. That would minimize the need to use any water. If anyone has to sneak in a toilet flush, just keep track of how many.
Once the fill is complete, go read and record the meter reading (take another picture), then lift the water use embargo!!
Calculate the difference between the two meter readings, 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 pool water chemistry adjustments, equipment adjustments, programming, purchases, etc.
Some meters are in gallons, others are in units. You'll need to know which and do the proper math.
The accuracy of the result will be based on how well you limited water use during the fill, and the water meter itself (typically accurate to about 1.5%). In your case ±500 gallons or so, plenty accurate enough for figuring out which SWG to buy, or how much chlorine, or CYA, or salt to add!