Does anybody know a good way of calculating pro-rata holiday entitlement for employees?

For example, one of our employees is leaving the company and her last day of work will be the 17th September. Our contracts of employment state employees are entitled to 26 days holiday per year or 2.17 days per full calendar month worked. How would I now calculate the entitlement for September? Unfortunately, the contract does not say anything about this and I am afraid it has been handled very randomly in the past.

I would be very grateful if anybody had any practice/ideas they would like to share with me.

Thank you very much in advance!
Anna Goliasch
Anna Lisa Goliasch