I cheat a little for figuring out interior coatings – The on floor square footage (3800) x 3 (for 8 ft ceilings) divided by 300. This is 11400 (3800×3) / 300 (average sq/ft coverage of paint) equals 38 gallons. This should be a close figure for spraying and allows for spray losses, overspray.
Many things can affect the amount of paint needed. The texture of the walls and ceilings, closets and shelving, raw texture needing primer, etc, etc.
The 38 (40) gallons will work for per coat estimating for PVA wallboard primer and paint. For repaints you will have to judge each room separately, length x width x height. The above cheat formula assumes one color and is a good way of checking yourself when measuring prints. If the LxWxH numbers are close to the cheat then everything is O.K. For 10ft ceilings use 3.5 instead of 3.