What most people have trouble with G112 is the C axis as it is not radial axis but a linear (straight line) axis.
Look at it as a X,Y application on a machining centre subsituting the X for C
The other issue I find is the tool compensation when using this function is a pain in the butt.
I always program the cutter path (centre of tool) as it is a lot easier
If you look at the sample program in the star manual the Y axis is still treated as a diameter and the C axis linear is a radius axis.
The C zero and Y zero is in the centre of the spindle