The rule of thumb I've heard many times is to take your rate as an employee and double it. The simplest way to do that is take your $K-per year and change the units to $-per hour. 100K per year become $100 per hour. I've also seen some stuff here on HN - and IRL - that suggests this is a minimum.
I saw in your other comments that you do mobile/full-stack dev - my email is on my profile and I do mainly backend dev work in Python and Elixir if you need any help.
I started using GAS at my day job to make a lot of my work easier. Thought it was fun and answered a few questions on /r/GoogleAppsScript/.
Got a PM from a guy in my city who saw my posts. He asked if I could help on a project he had. He's ended up giving me steady work for the past couple of years. The money's nice but it's a really good project and I'm more proud that it helps him run his business and that it makes life a lot easier for hundreds of people every day.
Did a few other small things for other people but all through referral from friends. I haven't really ever actively looked for work. Between day job, hobbies, etc. I don't want to take on a lot.
