[NTG-context] \let \def \relax ?

Hi, for reasons of consistency I'd like to define a kind of master-line. So I have everywhere the same proportions even when changing this master value. My idea was \def\MyGrid{15pt} and to use it for example like width=1.5\MyGrid But this doesn't work (error massage mentions problem with units). What is the right syntax for definitions like this? Steffen P.S. I'd like to use it in ConTeXt, but I guess this way of defining is pure TeX, right?