How turtleback armour adds to belt protection

Can the diagonal part of a turtleback armour scheme act as a second belt?

To an extent; it adds another protected surface, so judging the ship only by its vertical belt is incomplete.

The sloped deck in a turtleback scheme can contribute to protection after a projectile crosses the outer belt. It can act in part like a secondary belt or disrupt the projectile's path, although the effect is conditional. Comparing only vertical belt thickness therefore misses part of the protection scheme.