You've got to set a base rate to cover your expenses. Travel, gas, food, utilities, equipment, materials, whatever else. Then add a rate for your time and labor itself.
Edit to add:
I wouldn't charge by sqft until you're knowledgeable about how long it takes to do different size jobs. i.e. a 10x20 landscape typically takes ## hours, while a detailed portrait the same size takes more time. Then you'll be able to say, ah yeah that wall is so many sqft and you want this style, it'll be this per sqft. Which should be pretty much equal to an hourly rate. Then if you get faster and faster, you'll be able to make more money off the sqft price.
Good work though, have seen that video before.