In my country, I'd have to earn €38k before tax to keep €25k after tax. Does that mean I should actually plug €38k into FireCalc?

It depends entirely on taxation. Since it sounds like a large chunk of your €800k is money you've saved up, you've likely already paid tax on that money and it's not likely to be entirely taxed. That'd be like suggesting you owe taxes because you took money out of your savings account.

A lot of this information is US-centric, and a lot of people have their funds in a tax-advantaged account (401k, IRA) and they never paid any income taxes on it. So the entire amount when withdrawn is subject to income tax.

If you're living on investments held in a taxable account, far less of your withdrawals is taxable.