How do you figure the square footage of a dome?

Tricky. You need to flatten out the surface to work it out. Depends on the shape - if it's like a hemisphere then you can use the properties of a sphere - volume = 4/3 Pi R3 and surface area = 4 Pi R2 so if your dome is like half a sphere then its surface area = 2 Pi R2 and you can measure the radius R so can work it out!