The default ratio is 1/5th for domed planets vs. non-domed planets. This
implies that the dome covers 1/5th of the habitable planetary surface (and even if it's not a direct ratio, the dome would cover some resonably
HUGE area). Well heck, that's a lot of sheet metal!
Now, most obviously people would build underground instead of building gigantic (fragile) domes (but then we should add underground facilities, so that we can free up some space on those habitable planets too).