Deriving FIRE number - correct thinking?

In today's dollars, we need a total NW of $1.2M. That's 200k to purchase a home outright and 1M to use in retirement with 4% SWR. We would like to get there in 18 years.

If inflation is 3%/yr, the inflation factor is 1.7 - so our 1.2M needs to be 2M by then.

We currently have 100k for a down payment on a 600k home, and 110k in a brokerage account.

In 18 years, our 500k mortgage should be 390k (based in Europe, amortization schedule is different).

Simultaneously, our 600k home should be worth 1M at 3%/yr. The 200k home should then be worth 340k (perpetual tax deferrment means no tax on the profit used to purchase another home), and the remaining 60k profit would have a 10k capital gains tax, leaving us with 50k cash profit. Plus the 210k - so total equity 260k.

The 110k brokerage should grow to 440k at 8%/yr. If we additionally contribute 1.8k/mo and increase those by 3%/yr, we should have about 1.45M. Plus 260k cash from selling the 600k primary residence, so 1.7M, the equivalent of 1M today.

Am I thinking about it correctly? How have you derived your FIRE number?