As you said, there are a lot of variables to roof mix vs. sq. footage. However, a good starting point in my experience is 1 gallon of roof mix covers 100 sq. ft.
I am referencing the square footage of the roof with pitch factored in, not the floorplan square footage.
So with a 5,000 sq. ft. roof, I use about 50 gallons of mix.
With a cedar roof, make sure you pre-wet the roof down with water first. This prevents the cedar from simply soaking up the bleach. Not only will you use a lot more chems, but the bleach soaking into the middle of the cedar will cause it to break down more rapidly. You want your mix to dwell on the surface of the wood. Filling it up with water first accomplishes this.
Our standard mix for cedar is 2-3% mix with 1 ounce of Terra Wash added per gallon of mix. Then we rinse with water, and apply Wood Restore as a final step. The Wood Restore is awesome! Just apply and leave it in - no rinsing required. It brings a nice color back to the wood and leaves in a UV inhibitor.