[NTG-context] Calculating with variables

Hello there, I am new in variables in ConTeXt, so I tried the the following example of two font-sizes wich should have a ratio of 0.7: \setvariables[fontsize][bigsize=17pt] \setvariables[fontsize][smallsize=0.7\getvariable{fontsize}{size}] \definefont[big][name:arial at \getvariable{fontsize}{bigsize}] \definefont[small][name:arial at \getvariable{fontsize}{smallsize}] \starttext \big Dies ist ein großer Text.\blank \small Dies ist ein kleiner Text. \stoptext That does not work, what is the correct syntax? [Second question about this will be much more involving I think ... :), but I will give it his own thread]. Thanks. Huseyin