[NTG-context] Scope of BodyFontSize etc.

Dear wizards of ConTeXt, I'm wondering why ConTeXt doesn't seem to know about "BodyFontSize" in the "MPinclusions" block … try: --- \startMPinclusions DefaultRuleThickness := 0.47998pt*BodyFontSize/12pt; pickup pencircle scaled DefaultRuleThickness; defaultpen := savepen; \stopMPinclusions \startuseMPgraphic{localRule1a} draw fullsquare scaled BodyFontSize; \stopuseMPgraphic \starttext \startformula \Phi(\useMPgraphic{localRule1a}) \stopformula \stoptext --- As soon as I move the content of "MPinclusions" over to the beginning of my "useMPgraphic" code everything works as expected … Where's the best location to put this sort of definitions then? Is it the "MPenvironment" block? Oliver