The problem is, it's not addition and subtraction. It's exponents and amortization and, sorry, most folks only think linearly. Beyond that they get frustrated and reach for the next beer.
You don't need exponents if you never go further in debt than your bank account balance. Just pay off your credit card every month. I do, have done so for years, and it is not a rule that is very hard to remember.
So you just need the "count the digits" rule of thumb to decide if you can afford to buy something or not. If you can count to 10, you could even compare the first digit of each number (credit card debt vs bank account balance).
