Is this not being overthought?
The circling area is designed to give a minimum terrain clearance, so making it bigger by including a day only runway will only, potentially make the circling height higher than if was excluded (at night).
So, I reckon any usable runway (even unlit) defines the area. Simply, there is only one mda and one area and it has to cater for the most limiting scenario, which would be circling to that day only runway.
The fact you don't use the 'extra' circling area at night just means the circling area that you do use is just slightly higher than it would be otherwise.
Af
Last edited by as fragged; 10th Feb 2015 at 09:44.