Upload a photo and click on the largest darkest area, the largest lightest area and the most average tone. Then see where they really are. Your eyes are easier to fool than you think.
Lightness is measured per pixel as perceptual L* (CIELAB, 0 = black, 100 = white). The darkest 1 % of the image (and at least everything within 5 L* of the darkest pixel) counts as the darkest tone; everything within 2 L* of the lightest pixel counts as the lightest tone. Pixels of the same tone that touch, or are at most a few pixels apart, form an area. The target is the largest such area. A handful of stray noise pixels (the extreme 0.05 %) are ignored when finding the extremes.
Your click is read from a small area around the cursor, picking the best tone there, so you don't have to hit a thin letter stroke pixel-perfectly.
Darkest / lightest: the largest area gives 100. A smaller area with the right tone gives 50 to 90 depending on its size compared to the largest. A tone that isn't extreme at all gives at most 50, less the more of the image beats it.
Most average: within ±2 L* of the image's mean lightness gives 100. Beyond that you lose 4 points per L* you're off: 5 L* off gives about 88, 10 off about 68, 15 off about 48.
On reveal the largest areas are tinted strongly, smaller areas with the same tone faintly. The dashed marker shows the target.
Shortcuts: 1 2 3 pick target, Enter reveal, M tone map.