I have checked it on several simple plans - white circle on black square, white triangle on black square, etc. etc. etc.
It seems to be dead on the money. The attached, for example, works out to 72%.
It's a circle inscribed within a square. The ratio of the surface area of a circle with diameter N to the surface area of a square of side N is about 78% (78.54).
Then there's a little square, 1/16th the area of the big one. 1/16 is 6.25%. 78.54 - 6.25 = 72.29. Photoshop reports 72%.