How do I calculate my returns for STI ETF if I'm investing into it each month via a Regular Savings Plan (RSP) & DCA?

I'm not sure how this "Face Value of Total Shares (S$) - Total Amount Invested Including Fees (S$) to get my Return of Investment (S$)." works cuz 'Face Value of Total Shares' refers to the original price of the stock to my understanding.

For instance, NIKKO AM STI ETF has fixed gross sales charge of 0.82% which you can multiply by $150 (example) per n number of months (sales charge). Then using Excel, you can track average price bought. Compare it with current price - sales charge and you get an idea whether you have paper gain/loss.

If you want to take into account of dividends, it'll be more complicated but you can just less from accumulating sales charge.