I know there is disagreement on this and CPAs might cringe at this method, but for some of us it makes perfect sense. I used to have a large potential net worth in unexercised stock options, and it was better to think of it in terms of how much it would be in post-tax dollars. From there it started making sense to think of everything in post-tax dollars.
If I convert 100K from a tIRA to a Roth and paid 20K out of pocket in fed/state taxes now rather than probably the same 20% later if I'd have taken tIRA distributions later, I really didn't change my financial state even though I now have 20K less in my account totals to show for it. I find this method is good for treating tIRA, Roth IRA, and taxable accounts with cap gains (or losses) on the same ground.
Also, while I'm mindful of managing taxes, I don't want to make choices just to limit my tax expense to some budget. I can't keep all my cap gains under the 15% bracket. If I have reason to sell something and take a large cap gain at 15% one year, I'm going to do it. Rather than have an uneven budget skewed by the occasional sale, I just keep the cap gains and conversion taxes out of my expense budget and instead treat them as a future liability on my balance sheet. I calculate my withdrawal rate on my assets minus the future tax liabilities.
I honestly don't see how this method is inferior. I may be able to manage some of my capital gains to be taxed at 5% (state only) rather than 20%, but I view the 20% reduction as being conservative. I can't be 100% accurate on that tax liability, but neither could I be 100% accurate in budgeting the future tax as an expense either, if I treated it that way. Just because I mentally and on paper took 20% off those gains doesn't mean I won't try to do better with managing it. I mean, if I'm house shopping I may plan on 300K for a house, but if I find one that suits me well for 250K I'm not going to pass on it because it's under budget, or insist on paying 300K because that was my foregone conclusion of what I'd spend.