Polar Homing and Firmware (no replies)

I saw the PiMaker today, which got me interested in polar printers. I was wondering how you would home the R axis...I'm thinking a little flag for an opto-endstop on the side of the build platform, or on the shaft which the build platform rests on....

But my main question is, what is the algorithm to change cartesian coordinates to XYZ coordinates, and how would this be implemented into the firmware?

Thanks!
evamvid