Why is dot gain commonly measured at 50%?

Tone value increase, or TVI, is a normal phenomenon in printing that causes halftone dots to gain, or become larger, from the original file to the printed sheet. For example, a dot that is specified as 50% in the page layout software will grow when it hits the paper, usually around 18-20%. Dot gain occurs at the edge, around the dot. Dot gain is measured and controlled in North America at 50% (60% in Europe) because midtone dots have the greatest perimeter and the most opportunity to grow and fluctuate on press. Above this, as dots contact one another, the perimeter available for dot gain is reduced.