What you charge your customer as self-employed dev can't be compared to what an employee makes. The rule of thumb I heard is that you should consider about a third to be "net income". The rest is taxes, investments and various costs (including insurance and retirement).

So a self-employed dev making 100k would be comparable to an employee making 30-40k. It's a very rough estimate; it's probably a bit better than that.

I'm self-employed and have about €120k revenue per year (working 4 days a week), and I think my taxable income ended up below €60k last year.