Sorry correction to that calculation. I'm on a rush-hour Japanese train
Should be:
End value - starting value + withdrawals - after tax contributions = gain for the year
(End value - $3M)/ End value = % y
Extra tax payable = % y x gain for the year x 15%
So assuming no contributions or withdrawals your extra tax would be
300,000/3M = .1
tax = .1 x .15 x 300,000 = $4,500
