group footer fixed-position

Hi

I am trying to position a group footer at the bottom of a page. ( report designer 5.0.1-stable )

I believe I need to set
fixed-position, String, (Group elements only) Shifts the band to a fixed position on the page

but what value should I use.
it seems to want a number but a small number <= 183 does nothing and a number > 183
crashes the report.

thanks