[NTG-context] non-integer bodyfont size

I'm a bit confused about font sizes. I had a bunch of missing \Delta symbols in inline math (e.g. $\Delta x$), which I tracked down to \setupbodyfont[10.5pt] The non-integral point size works for text but not for math (the \Delta turned into a backquote). How to fix that? type-siz.tex has several sections to which I could add \definebodyfontenvironment or \definebodyfont lines. Which sections would I add to? Or can I put the additions outside of that file? And, most crucially, what are the magic lines :-) Thanks for suggestions, which I'll wikify. -Sanjoy