You know how to charge your clients based on several things:
1) how much you NEED to pay yourself (first and foremost)
2) all other operating costs (and I mean ALL of them)
3) whatever profit you can righteously make over 1 and 2
There's no magic bullet for determining what you should charge.
There is one thing you should NOT do - and that is call around asking other people what they charge. I get dozens of calls per week from jerks, pretending to be customers, simply asking me what my "rate" is, or pretending to have work for me to quote. That's not professional, it's not honest, and it's no good indicator. (so please don't do that to othe people) That will only waste your time and theirs, because if they have a handle on their business, and know what to charge, they're already ten steps ahead.
My advice, from experience - look at median salary statistics for people in your field, in your area, and base your own salary accordingly. Factor in your expenses, estimate you yearly workload, and then derive your base charge. Re-evaluate it in a month or so. Once it seems like you're making money, re-evaluate every 6 months or so. When you know you're making money, adjust only when necessary.
Hope that helps.
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