In case of Provident Fund, 12% PF is payable on basic salary.
Salary is limited to 15000. That means regardless of your salary amount you can calculate 12% on 15000 and pay PF along with employer contribution of same amount.
Even if your salary is more than Rs. 15000, you will be required to pay a minimum PF on 15% of 15000 and not more than that which is optional.
For calculation of provident fund, Basic Salary and DA is considered.
For example if your basic salary is Rs. 20,000 than you have to pay minimum PF of Rs. 1800 (12% of 15,000).
You can not pay lower than decided by PF Act. You have to deduct and pay at least 12% of Basic Salary.