You can't do any calculations in the interactive schedules, other than simple summation of columns.
What you will need to look into is expressions in properties (search these forums and Archicad help).
You can associate a property to your elements )Walls and slabs).
This property can contain an expression (formula) that can calculate the figure you are after - i.e. multiply the surface area by the labour/waste factor.
You may need more than one property which is fine, and these properties can be used in expressions of other properties.
You can then schedule the resulting property rather than the surface area.
Barry.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10