Calculations in group footer

[markp201]

For a pixelperfect report, we have simple sum fields in the group footer but

we want to see the pct of total for each column.  For example

50	20	10	80
62%	25%	12%	100%

I'm using a formula but from the lengthy generation, I get the impression it's calculating for every detail row.

Can we reference a footer cell somehow?