A good rule of thumb is to go with hourly for large commercial projects and for large organizations (or combination of fix and hourly after certain threshold) while a fix flat rate accepted upfront for small businesses and startups.

You can calculate a decent (non-exagerated) hourly rate as if you were an employee somewhere doing that work and then add a multple of 1.5 or something for your business profit and charge that hour multiple as a flat fee for the project.

My name is Humberto Valle, I'm the co-founder of Unthink Digital Marketing - if you have a link with samples I'd really love to see your work. :)