Hi there,
If your walls are not intersected with slanted roofs, you could use the "Minimum Height of the Wall Skin on the Inside Face" field in the Scheme Settings of the Schedule. This should pick exactly what you need if the model is correct. I don't remember if this field is available in AC18, though.
Otherwise, in AC 22 take a look at Property Manager and the new Expressions. Again, the uniformity of wall heights and slabs will impact the accuracy of calculations.
arch. ernest atanasiu
AC 10-26 INT/GER/FR on Win 10/ Win 11