What's the fabfilter discount schedule?

It's complicated, based on the total price of previously-purchased products and the price of the next item(s) you want to buy. The largest discount is applied to the least-expensive item on your to-buy list. The maximum discount is 60%. They don't really explain it beyond that. And to further complicate matters, they've changed the rules at least once that I know of, which may be why they're reluctant to spell it out.