OK, for a gasser, or a diesel at the car/RV pumps with the "Frequent Fueler Card" program, each 15 gallon purchase gets you half a shower credit. It's that simple, no further calculations required.
A 14 gallon purchase gets you nothing, and doesn't count towards anything. A nine gallon purchase followed by a 21 gallon purchase counts as a single 15 gallon purchase. It's not about 30 gallons, unless you have a single thirty gallon gasoline purchase. It's all about buying fuel in 15 gallon chunks.
The way to "work it" is, whenever you pass by a Flying J and can fit 15 gallons into your tank, stop and do just that. Each 15 gallon purchase gets you half a shower credit and half a drink cup credit. Each half-credit expires after one week, at midnight Mountain Time. You can combine two half shower credits into a free shower, and combine two half drink cup credits into a free drink cup.
It's much like the loyalty program at Autozone, where each $20 purchase gets you one point, and once you reach 5 points you get a $20 credit. If you have a single purchase of less-than $20, it counts towards nothing. If you go in and spend $100, it counts as a single $20 purchase. That's why those of us with Sprinters who buy the $7 oil at Autozone do so three quarts at a time, since three quarts gives us a single $20 purchase. A gallon of Power Service and one quart of oil also gets you one $20 purchase. I recently bought windshield wipers, Rain-X at $20.99 per blade. I bought the 24" driver's side blade at one store, and the 22" passenger side blade at another store on the same day, two separate $20 purchases gives 2 points. If I had bought both at the same time it would have counted as a single $20 purchase and given me only a single point.
I'll usually go in and grab three quarts whenever I pass by an Autozone, but sometimes I'll forget, and I end up needing to get 10 quarts at the same time for an oil change. Last week on the same day I grabbed three quarts in Winnemucca, three in Sparks, three in Reno. Three separate purchases that gave me 3 points.
These loyalty programs are presented in a manner which, if you understand them, you can work them to your advantage.