Hi Vamsi,
If you are generating quotas based on 'Time Type' (Day balance in base entitlement); to achieve your mid hire proration you can follow below logic
i) If employee is new hire, create a time type X and query current date (CURDAY) if it is 15th or below set time type X =1, if day is after 15 then set time type X=0
ii) while generating accrual at the end of the month, query time type X and if X=1 then accrual should be 1 and if X=0 then the accrual should be 2. (optional: You can restrict querying X by checking if his seniority in months; if the seniority is 1 you can skip checking time type 1)