LOS discount - greater than 6 days but less than 28...with a CAP?

I have LOS discounts (6-28 days) 10% off regular rate and LOS discounts (28 day or more) 55% off regular rate. To stay for a week or a week and a half for example....its currently cheaper for a guest to pay....than lets say....the same guest was to put in a month (29 days). BUT when a guest selects for example...like 20 days....now they are actually paying more more!...for just 20 days...then if they selected 29 days. So what I am saying is how do you put a CAP on it? At some point, those days...that fall between 18-28 total days...it ends up being more affordable if the guest had simply booked for 28 or more days. But that doesn't make sense right? How do I control this? They should not be paying more. Should it be capped? or is there a trick that I am missing?

You just need to define multiple discounts. One for bookings of 7-10 days. Then a bigger discount for 11-20 days. Etc., etc. That way the longer they stay the greater the % off. As for the person booking 20 days paying more than if they'd booked for 29, maybe keep the rates the same for everyone and only apply the huge discount to days equal to or greater than 29. A lot of that will depend on how the rest of your market does things.