Leave Entitlement vs. Encashment
It is leave entitlement and not encashment. Encashment comes only at the time of separation of an employee's service.
Leave Calculation Under the Factories Act
Under the Factories Act, methods of calculating leave are different for employees already on the payroll on 1st January in the preceding year and for employees who join after 1st January in the preceding year.
In the case of the former, the employee will be eligible for 1 day of leave for every 20 days worked if they have completed 240 days in the preceding year. In the case of the latter, the employee will be eligible for 1 day of leave for every 20 days worked if they have worked for a minimum of 2/3 of the total days from their date of joining.
Of course, the rate of 1 day for 20 is statutory. You may give 1 day of leave for, say, 18 days.