Work backwards from what you want to take home to figure out exactly what to charge. Start with your life, then build your prices around it.
What you want to actually pocket each month after everything.
Add up your typical monthly business costs. Leave blank any that don't apply to you.
Tax rates vary by location and income. These are rough starting points for US-based freelancers/solopreneurs. Talk to your accountant for a more accurate number. This is just to get your planning started.
There are 52 weeks in a year. Account for vacations and time off.
Any task you can delegate for less than this amount is costing you money to do yourself.