It is not critical and probably too late now to measure your fill volume. Some have commented on other new builds that if you note your water meter when you start to fill and then after when pool is full you can get a good estimate of your true water volume. You would need to deduct standard usage such as x gals for a bath/shower per person, x gals for dishwasher, etc. Not critical but it does help to ensure you have a good estimate on your Pool Math app when you need to determine chemical additions.
You should measure time to fill to certain levels - like to bottom of last step. at level of bench, at level of second step, etc. All of that info should be kept so if you have to drain water in the future, you have some conception of time to drain down to a certain level and refill. Just helps to manage your time to do certain things in the future. Again, not critical but it helps in the future.