Dunno if it matters - they all work slightly differently and use water differently. If i fill up my Resmed tank and set the humidifier to the highest setting, it will use up around a bit less than half a tank in the night. When I used an s9, although the tank was visually smaller, same results. I assume with the A10 it is the same, too.
You can fill you PR tank to the line with tap water, then pour it in a measuring cup to measure.
Off topic comment: Evidently FFM's use more water than nasals since I barely make the night (7 hrs) with a full tank, if set at max. humidity.
The humidifications systems sense relative humidity and the water usage varies depending on the relative humidity at any given time in your bedroom or sleeping environment. I am in Denver area, high and dry, I will often use an entire tank of water during an eight hour sleep. If I vacation at sea level with same settings I will use considerably less water per night.
I even see variations from night to night at home depending on if it's a dry night or a rainy night.
You can supplement your system by using a room humidifier so your bedroom is more humid and then you'll use less water from your humidifier tub on your cpap.