Disable brightness keys on iMac

This is correct. If the Spyder said 120 after having done an ambient light measurement, then that is probably the correct brightness for how bright the room was at the time you calibrated it. If it just recommend it without doing anything, it might just be giving you an arbitrary value which is worthless.

If the brightness varies in your room, you should set it to how bright is usually is when you do most of your color critical editing (e.g.: if you mostly edit at night, do not calibrate with the noon day sun coming through the window).

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\~K