Stop with this nonsense!
There is no benefit in underfilling a tank. Because as a diver you will do a buddy check. Part of it is check the tank pressure. So, even before you dive, you know how much air is in your tank. You do not dive with a tank that has not enough pressure in it.
Actually you check the tank pressure before you even put your BCD on. You open the tank valve, then check the pressure and take a breath from the tank.
As I said, there is no benefit for a dive shop to not fill a tank properly. Nobody will start a dive with a halve empty tank. Now go to your drink buddys in the Bar and check that you don't get a halve full beer bottle. Thats more in line with your knowledge and to sell you a halve empty bottle would probalby benefit the Bar.