I think for most people here, the tax impacts are pretty minimal, especially with the new tax plan; between a $24K standard deduction for income from pre-tax accounts, and the favorable CG rates for any assets you sell, most are expecting to pay little to no taxes.
We are not really in the RE crowd, and we are not expecting a huge drop in tax bracket, so we just guesstimated an effective rate based on our experiences over the past decade or so and our projected future RE budget.* As it happens, we just recently took a closer look at our asset breakdown to figure out our likely effective tax rate (how much "no tax" accounts[Roth/cash] vs. "regular taxable income" stuff [401(k)/IRA/SS] vs. CGs [stocks/MFs]), and the guesstimate we had been working with was within about one percentage point of that more detailed breakdown.
Of course, it's all a crapshoot; even if I FIREd today, Congress could change the tax laws 10 times before I die. So my plan is more guesstimate + flexibility of multiple types of investments so I can adjust my withdrawals to whatever ratios are most beneficial at that time.
*Like the others here, I have never tried to adjust NW for taxes -- I look at it in terms of how much I will need to withdraw in any future year to pay taxes + cover my budget. But I guess you could just scale up your required NW by whatever you expect your effective tax rate to be.