Sorry. Didn't mean to be coy. But, to make the math easier, imagine a full timer 25 days of vacation per year = five weeks. Great.

Now let's say that person goes down to 80% time, working four days a week. Now they only earn 25 days * 80% = 20 days of vacation per year. But this person only works four days a week. So to spend a full week away only requires using up four vacation days. (20 days of vacation) / (4 days/week) = five weeks. Great.