^^ That's about right...
I definitely agree that twice your salaried hourly wage is a good number, and go up from there. If you happen to be a subject matter expert who can do the work in half what other people can... asking for a lot is totally fair.
You can also do fixed bid projects, if you trust the people involved not to scope creep (or you're good at pushing back on that).
It's not as relevant with a single company taking your services, but never be afraid to add a "You're a pain in the ass to deal with" factor when calculating wages.
Also, I've gotten in the habit of making my hourly rate "what I actually want + 15%" or so - and then adding a 15% discount for payment on net 15 terms. I used to have some clients who were really bad at paying me on time, and this seems to help. If they pay you quickly, awesome. If they drag their feet, you get more money to compensate for it.
Then, if you don't feel like taking work, you can add an "emergency rush" fee on top of stuff. "I'm busy with other stuff, and can get to this in 3 weeks. If you need it in 1, that's rush rates, +$50/hr."
... basically, raise rates until the amount of work you're getting matches your desired amount of work. In my experience, the number required to do this is radically higher than most people think it will be, and if you're good, there's an "annoying valley" where you raise your rates and get *more* work for a while.
And don't forget to save about 40% for taxes. Self employment taxes suck, but on the plus side, you can deduct a whole lot of stuff (computers, internet, cell phone, part of your mortgage/utilities/etc if you have a home office, miles driven, etc).