Freelancers spend real time on admin, marketing, invoicing, and finding clients that can't be billed to anyone. The share of working hours you can actually bill varies a lot by field and by how much time you spend finding clients — we don't have a verified figure, so track your own last few months and use that. Using your full working hours here would underprice your rate.
Use your effective combined rate, including any self-employment tax or social contributions specific to freelancers in your country — this is usually higher than an employee's tax rate for the same income. Check with a local accountant for your exact number.
That's common — many freelancers underprice by comparing to an employee's hourly-equivalent salary, without accounting for the taxes, benefits, and downtime an employer normally absorbs. Use this as a floor, then adjust for your market and experience level.