I don't, simply because my effective tax rate is a mystery. If I were to deduct most or all of my money from the year from a Roth IRA, my tax would be zero, from a traditional IRA, my tax would be nonzero. Of course I could also plan for it, maybe it would be 5%, or 10% if i had a mix of the two. Why try to impute an uncertain tax rate to your net worth. I prefer to recognize that my total assets are $1.8M and if I were doing the 4% rule I could withdraw $72,000 and then figure out how much of that is taxes and how much gets spent and see if that works.
If you have a pension that pays $1,000 per month, I would keep that separate and figure that my assets need to deliver that much less per month. But if you are using the 4% rule hard and fast you could take the $12,000 per year divided by .04 and treat that as an additional $300K net worth. But that would only work if the pension keeps up with inflation like Social Security. If it doesn't you might only be able to call it $250/mo or $500/mo in today's dollars if your time horizon is a ways out.