The practice mentioned in your query is standard and does not seem to be in violation of any regulations. The Provident Fund (PF) contribution is calculated based on your basic salary, which in this case is Rs 15,000. This is in accordance with the Employee Provident Fund Act. The act mentions that PF is calculated on the basic salary which includes dearness allowance, retaining allowance, and cash value of food concession (if any). Further, it is mentioned that the PF contribution is 12% of the basic salary, which matches with the Rs. 900 contribution for 15 days that you mentioned.
However, with regards to splitting the minimum wages, the Supreme Court has ruled that splitting of minimum wages for the purpose of PF contributions is not permissible. The reason being that it would lead to a lower PF contribution and thus defeat the purpose of the act. But from your query, it seems like your employer is not splitting your minimum wage, but only considering the basic pay for PF calculation.
In case you wish to increase your PF contribution, you can do so voluntarily by informing your employer. This is subject to certain conditions and your employer's approval. For more detailed information, you can visit
https://www.epfindia.gov.in.
In conclusion, your employer seems to be following the correct practice for PF contributions. If you have further doubts, consider consulting with the HR department of your company or a legal expert for personalized advice.