Thermostatic mixer shower drilling spindle

[miketurner201]

Hi
I have a concealed thermostatic mixer shower with three control knobs. The one that changes the temperature of the water (middle knob) just slips around and around and having removed it I can see there is no grip on the spindle and it's just created a groove all around. I'm aiming to drill slightly so that the grub screw can grip the spindle better.

My question is do I turn the spindle as far as it goes to cold then drill the underneath, where do I align the spindle before I start drilling so it will turn from cold to hot ?

Thanks.