That is not something you would typically set, used only for manual focusing for a specific distance using the lens focus control ring. It is particularly of low use in a lens like the f1.2 where focus is extremely critical so you won't be measuring distance from the camera sensor plane to the subject.
Canon's 50 and 85 mm f1.2 lenses are excellent but VERY unforgiving. Make sure that you have the aperture set narrow enough to keep everything needed to be in focus within focus. For example with this lens wide open (aperture f1.2) at a distance to do a "head and shoulders" photo, the entire depth of field is less than one inch so if a person's eyes are in focus, their nose won't be.
Typically you wouldn't use this lens set to f1.2 except in very low light OR when you need to make the background disappear into a blur (which this lens excels at doing). But when using its extreme low light and/or extreme shallow depth of field capability, your focus needs to be spot on and if you are using a DSLR then make sure to use Canon's microfocus adjustment (built into their later DSLR bodies) to perfectly calibrate that lens to the specific camera's AF system.
I love Canon's fast f1.2 primes but I am extremely careful using them because it is very easy to end up with a bad image by missing the focus point or using too wide an aperture resulting in insufficient DoF.
With a natural light (assuming good light) portrait, I would set the ISO to 100 or 200 for best sensor noise and dynamic range performance. Decide what you need in terms of depth of field based upon your distance from the subject and set the aperture accordingly. You can then either use aperture priority mode of the camera and let it set shutter speed for a proper standard exposure or use full manual and dial shutter speed in to provide a standard exposure based upon the camera's exposure meter display. If that results in too slow of a shutter speed to safely hand hold the camera without create blur to camera shake then either use a tripod or consider using a wider aperture if that provides an acceptable depth of field.
Here is a link to a decent depth of field calculator that will give you a good idea of how much of a subject will be in focus based upon distance to subject, lens focal length, and aperture: https://www.pointsinfocus.com/tools/depth-of-field-and-equivalent-lens-calculator/#{%22c%22:[{%22f%2...
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
