My gut on this one leans toward a flat project fee – from the client’s perspective I think that makes more sense. What I’ve done in the past is estimate the number of hours a project will take (leaning toward the high end) and then multiplying that by my hourly rate to come up with a project fee.
There will likely be some calibration over time as you get better at estimating how long a particular task will take, but that will come! And then you might even be able to price it this way: and infographic of size __ costs __. You could have a few different go-to sizes to simplify the process.
Hope that helps,