I am assuming you are referring to income taxes. If not, please ignore the following response.
Why would I need to add income taxes to the calculation of my annual expenses? Apart from interest/dividends that I hope to reap from my nest egg, and the resultant income tax I'll pay on that, such taxes are not an expense to me, particularly since I ER'd and no longer make as much as I did.
Let's pretend I have now looked over my past few years of expenses, figured in the new expense of individual health insurance premiums and other costs, and am predicting $30K annual expenses for the foreseeable future--which does not include any income taxes in my list of expenses. Let's also pretend that whether I plan to work part-time or have only interest/dividends as income, I'll pay taxes only on that extra income. Yet taxes are never 100% or more of income, but are less than a third of that income. In ER, it will be much, much less. Taxes are an expense, if you will, only on income I'll make. But I'll still have [income minus taxes=net $].
In summary, I just do not see the need to figure in income taxes when coming up with my annual expenses.