Don's schema seems pretty solid and scalable, but may be normalized a bit more than your needs, particularly regarding PTO Type and tracking history of Status changes? The many to many with a reverse relationship can be fairly challenging to wrap your head around.
I suggest keeping it simple, at least initially, and create a Summary field on parent table (Employees) that sums the child (PTO Requests) time. Then, subtract that value from the where you are storing the 20 day value on the Employees table. Watch out for your formula data types (duration vs. number), QB will bark at you and guide you to what it is expecting.
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Brian
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