I can give you a baseline to think about. First think about what the salary for that job would be. Let's make up a number for arguments sake it's 10,000 euros a month (in total). (NB: I know nothing about Salaries in your part of the world)

Now, freelancers have to work a lot of hours a week that aren't billable, for various reasons.

So - for arguments sake, lets say you can do, on average, 5 hours a day of billable time.

Let's also factor in public holidays, your holidays etc - so deduct 2 days a month.

Let's assume a month is 4 weeks, so 20 working days available, but minus the 2 so 18.

18d x 5h = 90h/month

So to make the equivalent of the salaryman, you need to charge 10,000 / 90 = 111 euro / hour.

Obviously you should adjust this to take into consideration any other costs you might have like professional indemnity insurance, accountants fees, business registration... whatever.

I hope that helps.