Was my friend cheated on her propane fill up????

My friend had some propane guy come and put some propane in her tank. It has a gauge on it..numbers read 0-100. I checked her gauge a day ealier and it had 30 on the gauge. Then when the guy added the propane the next day I checked it when he left and it read 80 on the gauge. He charged 2.58 unit price..I saw on the receipt she showed me. I also saw PREVIOUS READING 30.......after fillup. 80. Also I see a percentage sign there after each of those numbers. He charged her $206.40. But with this charge that would have been for 80 gallons. She already had 30 showing on the tank. So shouldnt she have been only charged for 50 gallons?? 80 - 30 = 50??? I told her to call the guy but he told her some excuse the price was the percentage or something. Even if he charged percentage e tank...at 2.58... for each percent she already had showing on the gauge 30..so shouldnt it been 80% - 30% . It would been 50%...so charged 2.58 x 50???