The way I've looked at this is to target a separate aggressive withdrawal rate for my IRA (401k consolidation fund). The idea is to always be drawing something from your IRA with hopefully the last dollar coming out the day you die. That way you can take advantage of the progressive tax rates for many years. For myself, I've targeted a 6% withdrawal rate (note that's not my FIRE withdrawal rate just the IRA withdrawal/Roth rollover rate). Assuming you're in at least the 25% bracket right now, and you're targeting the 15% tax rate for your withdrawals as you've mentioned, that would come to a 401k balance of around $800,000 based on current tax rates. Anything over that and you'll probably be needing to pull your IRA funds out into the 25% tax bracket. Still, that's not the end of the world since avoiding paying 25% now and paying 25% later still allows the balance to grow tax free until you begin withdrawals. When you add in the fact that you're probably also paying state income taxes now (that you can shelter in your 401k as well) that you should be able to avoid later by moving, it's pretty much a slam dunk to be always maxing your 401k if you have the option. But if you're thinking you need to get some money into your taxable accounts and the 401k is pretty much taking every last dollar you can otherwise save, I feel that $800,000 number is probably around the point where it starts to become a wash for 401k vs taxable (for the tax brackets listed in the example here).
Edit: sorry, forgot the part where you're married. So double the numbers. $1.6M for a married couple is your break even point.