20 lb / 4.6 gallons = 4.4 lbs per gallon
4.4 lbs = 70.4 oz
323.84 oz per tank
$15.00 per tank = $ .05 cents per oz
Weigh your tank each day for a week, document the weight each day, at the end of the week find the average usage and calculate your propane cost per day.
For example, if your average daily weight is 32 oz then your daily cost is $1.60.