Return Accrual Equivalent

Anyone know why I just can’t take the Pre Cap Gains Future Value of the portfolio and multiple the weight of the deferred long term cap gains, subtract the cost, multiple that by the cap gains rate and subtract from the Future Value to arrive at an “all tax paid” FV of the portfolio. When I do it that way, I dont get to the same answer…in theory anyone know why?

Would you have an example with some figures so that we understand what you are trying to do?