Ok. The following information can be taken out of the R2C2 schematics from BitBox:
     
http://ftp.o2s.ch/RepRap_BitBox_Wolfgang/calculus3.html

With this calibration and connecting the two power resistors into series it now shouldn’t start smoking anymore.
Well. At least as long as you prevent Ari from turning the heat onto 240°C again >_>