Thank you. However your 8 night itinerary is an easy one to figure out because it falls neatly within a couple rules of thumb for where the two EC nights fall:
FIRST EC: The first EC is never on the very first night of the cruise because there is (or was, back in the day) an assumption that it might take time for you to get your luggage (or locate it if it is missing), get unpacked, stuff ironed, get settled in, etc etc. Therefore the first candidate for EC is the 2nd night of the cruise. And if that 2nd night of the cruise is a sea day, then that is the answer for sure.
LAST EC: The last EC is never on the last night of the cruise because there is an assumption that your nice clothes have all been packed away in your suitcases, and those suitcases should be sitting out in the hallway waiting to be taken away. Therefore the first candidate for the last EC is two nights before the end of the cruise. And if the that night falls on a sea day, then that is the answer for sure.
The two itineraries below show the first and last EC nights falling neatly within the rules of thumb--2nd night (sea day) and next to last night (sea day):
What is less certain with the above logic is if the above nights fall on a port day. While it is preferred that EC nights fall on a sea day, it is not always practical to do that especially with a 7 night itinerary. However a longer itinerary affords the option to find a sea day to schedule an EC in the event the above rule of thumb puts the EC on a port day (below).
A final random fact is that New Year's Eve is always an EC night. On the longer itineraries, NYE becomes the 3rd EC night as you can see in the three itineraries. This may also be true for Christmas Eve but I don't have enough experience to know for sure.