I am trying to figure out how much propane I use per week and I am not sure it my calculation is right or not. We have a 500 gallon tank and they fill it to 80% so 400 gallons. By tracking how many weeks it has been since I last filled it to 80% and the percentage that is in the tank now it ruffly comes to 1.25% a week. Now to calculate that gallons per week I am using do I take 0.0125 x 400 gallons or 0.0125 x 500 gallons? Either way I am between 5-6.25 gallons per week? If any one can help I'd appreciate it.