Using giftables to round up after a certain number of items

We rent out glasses for events. Glasses come in boxes of 25, but if a customer orders 20, we just round up and give them the extra 5 for free.

Giftables looks like it might be able to do this for us, but would we have to make a new rule every time we go past that 25? So for example:

If total = 18 then add 7, if total = 19 then add 6, if total = 20 then add 5, and so on. This could get pretty laborious, as sometimes our customers order thousands of glasses, and to have a rule for every single possible amount doesn’t make a lot of sense.

Does giftables offer a more sensible way for us to achieve this?