This is exactly it. They simply divide what you paid (including taxes) last time you renewed by the number of months you subscribed for and give you that amount when you swap for credit. I think I get just over $46 in credit when I swap my 3-month. I usually swap my main and just pay for an add-on if there's something I really want. Or, if it's a single LP that I want and it is available for purchase in the store, I'll just swap for the $46 credit and pay $37 for the single LP when the credit shows up in my account at the beginning of the month.