In past I know that table calculations weren’t allowed for totals. See [retired]Why don't my table calculations have totals? .
I don’t quite see why table calculations couldn’t simply work on total rows in the same way they work on regular rows. Is it because some table calculation formulas would work on regular rows but return misleading results on total rows? I could see the mixing of null and non-null values being tricky for calculations using mean() and others, but SQL has the same issues and deals with them. Admittedly, sometimes the way SQL deals with them is very confusing to users and I could imagine that Looker would prefer to err on the side of caution.
Or has this changed?