OK, I'm finally getting back to this. I want to get an overall ballpark estimate of our YTD growth - don't have time to track monthly investments from 401k and enter it into an IRR spreadsheet. So, I tried to use the formula above: ($start-$now-$addedSinceStart)/$start
When I put my numbers in, I am getting a negative return, which doesn't make sense to me since the account has grown more than what we have invested (total invested this year so far is $37,951 and the overall balance has grown by $64,703, so our net growth for the year is $14,752). So the return should be a positive number. Shouldn't the formula be: ($now - $start - $addedSinceStart)/$start
($Now-$start-$addedsincestart)/$start is more correct than the first formula but its still not correct. If you have a large balance and small contributions it will be close-ish....but the larger your contributions are compared to your initial balance, the farther away from correct this formula will be.