Calculating Part of Fortune

Belatrix

I have one question about calculating Part of Fortune.It is supposed to be different for those born during day and those born night.But what is actually ment by this?Is nocturnal birth the one when Sun is below the horizon,or is it when Sun sets from MC down to IC,in other words during PM time?
One more thing.For diurnal births,we calculate part of fortune by adding the distance of Moon from Sun,to ascendant.In nocturnal charts,it is vica versa.
Lets resole this confusion,because I see that in a lot of computed programmes for astrology part of fortune is calculated in the same manner,no matter if charts are nocturnal or diurnal.