PTO Calculations

I'm trying to create a spreadsheet to help people track their PTO time. Here is the problem I'm running into, employees get 4.67, 10, 13.33 or 16.67 hours per month depending on how long they've been employed with the company. I'm not sure how to convert that into something we can use. Ideally, I'd like to have something that they can put the time off they are requesting and then each month they could copy and paste a formula to add the new time they've earned. Some employees are getting confused with what they are earning and they think it should be (5 hours and 7 minutes) which it is not. I'd love it to look something like the attachment.