Fast Formula Entry Validation to restrict 10 days request per year

Hi,

We have an absence type of 'paid sick leave' (qualification plan) which connected to few absence reasons. One of the reason is 'Sick leave - without doctor certificate'.

We want to limit the request for submissions related to the above reason to 10 days per year.

I thought about two possible solutions:

1. Use GET_VALUE_SET - the problem is that it seems that I cannot use group by and sum function in the value set.
2. Use array to loop through the requests which have been submitted this year.

Regarding the option two, I tried the below formula but it doesn't work for me.