Let me put this another way, by measuring from Dengar to the Decimator, from closest point to closest point, did he not also, by doing so, establish that the target was outside of arc? Did he therefore not fullfill what you are asking for:
- Measured range (for a turret)
- Measure arc: he is outside of arc.
He might not have answered the question of whether a shot in arc is in range 3, but he is not required to.