To my knowledge it's:
You calculate according to 2 parameters:
1)Number of pax
2)Number of doors
No CC if there are less than 20 pax
20 to 50 pax= 1CC
51 to 100 pax= 2CC
Then 1 additional CC every
1 CC for each pair of doors, plus 1 CC every 50 pax (even if quota of 50 pax incomplete= 320 pax: 7 CC )
Then 1 CC each pair of Type I doors. In case of odd number of Type I doors, the number of CC is rounded down (ex 3 Type I Doors= 1 CC)
The minimum number off CC required will be the result of the most restrictive of those two rules
ex. 350 pax, 8 Type I Doors:
Pax rule: 7 CC
Doors rule: 4CC
= 7 CC minimum (pax rule is the most restrictive )
In case of an aircraft with 2 decks, they have to be calculated as if it were 2 different aircrafts, unless one of the decks only has 1 Type I door and less than 20 pax.