Set the iMac’s brightness to the level you actually plan to use for photo editing before you calibrate. If the default brightness feels too bright, lower it first.

The key is consistency: calibrate the display at the same brightness you’ll use later when editing. If you change brightness after calibration, the calibration will be less meaningful.

Also, follow the Spyder software’s on-screen instructions. Some calibration software recommends starting around the midpoint (about 50%) brightness level, then calibrating from there. If that level matches your normal editing setup, it’s a sensible choice.

So in practice: choose a comfortable editing brightness, leave it there, and calibrate at that setting.