That doesnt look like 500 gallons! Looks awesome though. Can you put something by it for a size reference?
Finding and average is quite easy though, but you have to know the measuremets.
Say you have a set of 6 different numbers (2, 23, 45, 15, 78, 45) you add those together (208) then divide by the amount of numbers you have (6), your average would be 34.67. You generally use this to find averages for business use (like sale trends, profit trends, ect.).