First, yes. This is the ideal forum for discussing your question.
I don't see that there's much difference between being in the "webmaster" business and the post production business -- except for the fact that the post business requires that a much larger dollar amount of equipment needs to be maintained.
As you can read from any number of posters below and in the archives, you have to calculate what you personally need to live, what you need to save, what you need to have for recurring expenses (rent/mortgage, utilities, auto, accounting, legal, etc), what hardware/software configurations you need to keep current and last, but far from least, what you have to set aside for taxes.
Once you've added those up, come up with a realistic number of hours you can bill for in an average month (which does NOT include the time you'll spend marketing your services and doing general business stuff) then divide the costs by the hours to get a STARTING point for figuring out what your hourly, daily, weekly billing rate should be.
Of course you also have to be aware of your competitive environment. If your client knows the college kid living in his parent's basements is willing (and reliable enough) to be his webmaster for $15 an hour, your business may not be viable. In that case the value you add, the number of prospects you have and many other factors come into play.
One last thing, and I don't mean this in a mean-spirited way, if you're going to be doing a company's social media you will have to be very careful of grammar and spelling. The little stuff matters when it's someone's public image.