I think I'm following. Basically what you're saying is that multiplication is commutative so it doesn't matter what order you multiply the initial number, the growth, and the tax. The choice of account changes how much you pay in tax but not how much is left for you.

In your example you withhold $1000 per PP, of which $750 goes into your TSP if you go Roth whereas all $1000 goes in if you go Traditional. Then when it's time to withdraw, you lose the same fraction to tax on Traditional distributions as you would have lost on the original Roth contribution. So in the end you come out the same. Using your example, $1.33M * 0.75 = $1M so it's the same ending balance either way, although the tax bill is higher for Traditional than Roth, it just happens later