Attached is a screenshot showing the surface area that you want (paint, drywall) using the interactive scheduler.
The walls are 6" thick, 10' tall and 10' long. With the reference line on the outside as drawn, the interior surface that you're looking for is the surface area of the side opposite the reference line.
You can do something similar with list schemes, but I'd use the I.S. unless you have some reason not to.
Karl
AC 28 USA and earlier • macOS Sequoia 15.3, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB
One of the forum moderators