From ConTeXt wiki

In the version of ConTeXt that I am currently using (from TeXLive 2007), there is no leftmarginwidth setting (but rather leftmargin) and no headerheight setting (but rather header) etc. I only found out what all the correct terms should be by turning on visual debugging – which gave me a list. Is this page likely to be updated? (As a relative newcomer to ConTeXt, though not to TeX, I'm wary of making changes myself.)