It doesn't matter... you are falling into a common trap/popular misconception.
Say you have $10,000 in an tIRA and $2,000 in taxable accounts and your tax rate is 20% and your investments grow at 7% annually.
One choice is to Roth convert where you end up with $10,000 in a Roth and the taxable account is used to pay the $2,000 tax bill. After 10 years at 7% the $10,000 Roth has grown to be a $19,672 Roth that can be spent.
Alternatively, you don't convert. Over the 10 years, the $10,000 tIRA grows to $19,672. Meanwhile, the $2,000 taxable account grows to $3,449 (growth is less because each year's 7% get's taxed at 20% so the after-tax growth is 5.6%). If you withdraw the $19,672 tIRA to spend, pay the 20%/$3,934 in tax, at the end you only have $19,187 to spend ($19,672+$3,449-$3,934).
You actually come out ahead with the Roth because you avoid tax on the taxable account earnings... so where is this alleged growth that you are so concerned about?
Let's say that there isn't tax on the taxable account earnings, so over the 10 years it grows to be $3,934.... after you withdraw the $19,762 tIRA and pay the 20%/$3,934 in tax you have $19,672 left to spend... the exact same as the Roth.
So the Roth comes out better for the tax avoided on the taxable account growth if you don't convert.
And... this all assumes that the future tax rate is the same as today's tax rate... if today's tax rate is lower then you come out even further ahead... run the numbers for yourself and see.
So this simple example proves that you come out ahead even if the tax rate is the same then its pretty easy to see that if the tax rate is lower today that you come out further ahead... no complex models needed!
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$19,672 = $10,000*(1+7%)^10
$3,934 = $2,000*(1+7%)^10
$3,449 = $2,000*(1+7%*(1-20%))^10