well, obviously you don't re-order your own list, so it's not like a 2000 hire with a captain bid moves up, nor does a 91 hire flying 767 FO move down. But, you can look at it and say generally the 88 captains at Delta go from number x to number y, so you order the new list so that x-y end up roughly with the captains of an equivalent airplane from the new company. I don't care when our junior captain was hired. Consequently I think that person should end up approximately on the list to where he/she can still be the bottom narrowbody captain.
Every list has it's anamolies. The very junior person who got a captain bid and the super senior person still flying 767 FO so they can be number one in their category. So I still think you can go pretty much by equipment.
It really would not be much of a player in either a NW or UAL merger, since the ratio of widebody to narrowbody is pretty much the same. So a straight percentage in these sitiations would probably not be far off. But in a case like USAir and AWA, I think it reasonable that the USAir widebody captains go to the top of the list since AWA had on widebodies.