[NTG-context] Space from bottom footer to bottom rim of paper

I want topspace=bottomspace=2cm It works for topspace but not for bottomspace How to do ? Thanks Fabrice \setuplayout [header=26pt, headerdistance=25pt, backspace=2cm, width=middle, topspace=\dimexpr 2cm-\headerheight-\headerdistance\relax, footer=16pt, bottomspace=\dimexpr 2cm-\footerheight-\footerdistance\relax]