Thanksgiving day is a bad time to find out you grabbed the wrong color when you bought paint.
I thought I had an extra 5 gallon bucket but I only had about 2 gallons of Ultra Pure White. I put it in a smaller diameter bucket, took the strainer off the pickup tube on the sprayer and tilted the bucket back so the sprayer could suck the bucket dry.
I got two coats with about a pint to spare.
Pitch on existing roof?
Wall height to FFE?(finished floor elevation, be it a deck, the ground a patio, ect)?
Pitch on extension?
I assume you are coming off of a straight run of roof with no gables or valleys?