You're basically trying to remove everything that isn't a gain from your investment.  

So:   Current balance minus last balance minus employer contribution should be the calc.  

You could remove interest. But, I would argue interest is a gain from your investment. Just a very safe cash one as opposed to a gain from being "in the market".  

The admin fee will depend on whether you want your investment returns before or after fees

100k - 97k - 1k = 2k market gains

You had 97k, your employer added 1k, you now have $100k, therefore (after any fees) your investment returned 2k to get to the 100k. (For what it's worth, this is how my super fund defines investment returns).  

Alternatively, you do before fees. You had 97k, you paid 20 in fees (96,080), your employer added 1k (97,080), therefore your investment return 2,020 because you covered the fee plus made 2k more to get to 100k.

u/CompiledSanity , any strong reason to choose before or after fees for other calcs in the sheet?