Last year I worked for 210 days and I made 143.480€ (=85€/h) after deducting the 19% VAT. I paid 10.740€ for health insurance (which is capped) and 15.720€ for the pension fund (which I don't have to do but choose to do and this is the same rate an employer/employee would pay given that income) leaving me with 117.020€ taxable income. The income tax then is 117.020€ * 0.42 - 9.136€ = 40.012€ and 2.200€ Soli, leaving me with 74.808€ net income.
When you are employed then your employer has to pay half of your health insurance and pension fund so to have a net income of 74.808€ you "only" need to make 131.940€ per year or about 78€/h all things equal. So we are talking about a 10% difference. Now you could argue that an FTE would not work for 210 days but 200 or 190 days which would add another 5-10% difference but it would still be off from your original 33%.
To put this into perspective: I have been offered full time employment at almost every company I have worked for as a freelancer so far and the by far highest offer I got was 93.000€/year which is around 54.600€ net income - or about 20.000€ less net than I make right now. That is 2/3 of the average FTE income in germany.