If I choose to withdraw of my living expenses from account A, I would enjoy 35k tax free living lifestyle... but then later in like when I start to withdraw the same 3.5% from account B I would have a diminished living lifestyle.
How do you flatten the curve without doctorate levels of maths???
If your goal is to simplify the math, then simply withdraw 50% from each, considering the taxes, and call it good.
But really, taxes aren't that static, nor is your spending. I would plan your budget as if you are always going to withdraw from the account with high realized gains--the worst-case scenario. Then, if you have large unexpected expenditures: house repair, car purchase, relative support, etc. plan on taking that from the more-tax-favorable account, so you don't disrupt your plan. As the "primary account" gets drawn down, you may face changes in your plan, but they will all be favorable, rather than bad news or emergencies.
In the US, there are 3 buckets: a taxable account, which is kind of a misnomer, because the rate on the first $47k (or $89k if married) is 0%, if you don't have other income. Then there is the tax-deferred 401k or IRA, which is taxed as regular income as it comes out. Finally, the post-tax Roth account, which is not taxed. There are a wide variety of ways to combine these 3 buckets into a blend. This is not just to provide you with your spending money, but also (in the case of twx-deferred) to avoid mandatory withdrawals that could cause taxes to spike.
The fact that it is complicated is just another hurdle to FIRE. If it isn't something you want to do, then hire a tax professional to help you through it, and add that to your planned expenses.
