What is the calculation for subtracting the MER from the total? I didn't quite understand that part.

Yes, it is good to define TLAs (Three Letter Acronyms) when using them because people may assume the TLAs mean different things.

I'll assume MER here is Management Expense Ratio as described in

https://en.wikipedia.org/wiki/Expense_ratio.

Ignoring taxes, an investment compounded annually increases by (1 + i)^n, where i = annual investment return and n = number of years. E.g., after 30 years at 2%/yr, the investment would be worth 1.02^30 = 1.81 times as much as the original amount.

Instead of saying "...increases by (1 + i)^n" we can say "...changes by (1 + x)^n". Here, n is still years but x can be positive (as in the investment return above) or negative (e.g., a MER). An investment earning no return but subject to a 2% MER for 30 years would be worth only 0.98^30 = 55% of the original amount.

An investment earning 2%/year and subject to a 2% MER, after any number of years, would be worth (1 + 0.02 - 0.02)^n = 1^n = exactly the original amount.

In other words, the MER gets subtracted from the raw investment return to determine an effective investment return. Does that make sense?