I saw an example in official website, China Construction Bank, about your current situation. You can refer to it
If Mr. Li's bill date is the 7th day of each month, the total payable amount of the monthly bill of 65438+10,000 yuan, and the bill details are10,000 yuan. Mr. Li repaid 8000 yuan on the due date. As he fails to repay the loan in full, the billing bank will deduct the interest 122 yuan in February, and the calculation method is as follows:
10000× 0.5 ‰× 22 (22 days from10/27)+2000× 0.5 ‰×12 (65438+127)