It's really not that complicated.

You're going to want 5 years of expenses in your taxable account plus your Roth contributions when you retire.

You already have X years expenses in your taxable account plus your Roth contributions now.

Pick a date to aim for, maybe on the aggressive side: 8 years from now.

So this year, you need to contribute (5 - X) / 8 years worth of expenses to your taxable and your Roth.

Repeat the same math next year.

Yes, it won't be exact...but it will be close enough.

So say you spend $20K per year and have $30K in taxable and $30K in Roth contributions. That means you already have 3 years of spending in your Roth pipeline. Wanting to retire in 8 years, you would need to put away (5 - 3) / 8 * $20K = $5K in taxable or Roth this year. Doing that for the next 7 years will get you a total of ($5K * 8) = $40K. $40K plus the $60K you already had gets you to $100K, or 5x annual spending. Presto.

Over the next 8 years, your taxable account may grow or shrink due to market gains or dividends. You may change your timeline to be more aggressive or more relaxed. But as you get closer you'll be able to see more clearly what you need, and you can adjust from year to year.

(Also, because of the way the timing of Roth contributions and conversions and withdrawals happens, note that you might not really need 5 years of expenses in the Roth; it could be as little as 4 years. But how much you'll need exactly will depend on when in the year you retire, which is something you won't need to think about until you're a year or two away from FIRE.)

Good luck!