1) Create a virtual value device, eg: celcius
2) Create a blocky rule that looks like: http://screencast.com/t/79gv6vVGy
The top line puts the farenheit in a parameter, the middle converts it to celcius and puts it in a variable “item” and the bottom one puts the celcius in a virtual value, so it can be displayed and easily reused.
You can also choose to keep it as a variable if you only need it in blocky.
