Lets assume I want to have the average income of an office employee (in germany thats ca. 40000 EUR gross - german wikipedia)

• So, we start with 40000
• As I am a freelancer, I have to add the bits that an employer usually caries (germany: 32% - german wikipedia), so now we are at 52800
• To that I have to add cost for running my business (travel, laptop, phone, tax consultant etc.). Lets say 6000 EUR
• So, yearly turnover would need to be 58800 EUR per year
  

Now, lets make this turnover

• Assuming 2 months for holidays and sickness, this money needs to be earned in 10 months
• Within each month, paper work, client meetings, learning etc. must be done. So, I assume the amount of billable days amounts to 13 days per month.

Hence: 58800 / 10 / 13 = 452 EUR per day.

Here a little table that shows the numbers for different gross incomes (leftmost column) and the resulting day rate (rightmost column). Also check the linked spreadsheet if  you want to modify numbers.