Retractable cable management

how about enough cable to let the head go around 1 full turn
and then in the postprocssor (that makes the gcode) you prevent it going past 1 turn and 'rewind' to unwind the cables to continue the turn from the other side.

or sliprings to connect the cables through the rotating joint.

 

I think you can mean 2 things here, 1) how to find a home position, and 2) how to calibrate the distance moved

1)
if you use the 'fake the home' princicples you will not have to recalibrate often so long as you follow the procedures.
assuming a GRBL controller, GRBL sets home when it is turned on or reset.
knowing that, we can make sure the mechanism is aligned before we turn it on.
after that setting worjk coordinates it a known offset from the home and is easily repeatable IF you always park it at home before turning it off.
A simple macro can do the parking for you
G90
G53 G0 X0 Y0 Z0 A0
would return all axes to the home position.

2)
to calibrate the actual rotation angles is much the same as calibrating linear axes, you need a mark and a reference, then you drive the axis a known 'distance' and compare the angle actually moved to the expected angle and recalculate the steps/mm value
newstep/mm = currentstep/mm * commanddist / actualdist

for example a 200 step per rev motor with 8x microstepping will do 1600 steps per revolution.
regarding it as a linear axis the command
G0 X1600
should turn it one turn exactly, but if you have belts or gearboxes you may not know exactly how far it will go, then you measure the actual movement and compare it to the expected movement using the formula above to correct the steps/mm value.