The amount of LPG gas needed to refill into the storage tank all depends on the guage level of the storage tank. The appropriate litres to refill inti the storage tank is determined by the capacity of the tank. A 3,000 litres storage tank can only contain 2 tons max. Overfilling beyond that can lead to leakages, which may cause fires or explosions. So, 2 tons is equal 3,500 litres.
Before placing an order for a new refill, make sure you empty the storage tank. Drain the remaining LPG inside an extra gas cylinder bottles (50kg). By empting the storage tank before a refill, gives you accurate percentage level based on the tons purchased. This will also prevent scam from suppliers. Even if your LPG supplier manipulate the truck meter, the storage tank guage will indicate the level of LPG gas presently.