When you freelance, you're likely not receiving equity in your client's company, getting paid time-off, or health insurance. So it's on you to account for all of these things in your rate.
Research full-time market rates for your area
Services like LinkedIn, Glassdoor, and others allow you to find such numbers. Take into account your experience level. If you're still junior, don't use a \Senior Design Director\ salary in the following steps.
Divide by 52
The resulting number is your weekly rate.
It sets up better expectations with you and your client. You stop forcing out those 2 hours of \work\ at the end of the day. Your client doesn't feel the need to micro-manage you (imagine how they feel when you go to the bathroom. Literally $ lost). Instead, set weekly expectations that you know you can meet. Manage your time wisely and always deliver.
Take the base salary, then 1.2 - 1.5x it
The salary you found in the previous step is for full-time employees, but you're not. Multiplying by 1.2 - 1.5 accounts for the things I mentioned above (no equity, no paid time-off, no health insurance).