pretty sure having the 5th in the group doesn't matter.

I don't know the equation, but I can tell you, I get about 550-625 per kill as a 57 in ramps. My 4 shaman, and 1 70. When the 70 zones out before the kill (thus 4 people in, 4 x lvl 57) they get 900-950

Also, if I have just 1 shaman in, and zone the 70 out, the 1 shaman gets 2780 per mob. This was @ 55, so I am sure it's a bit higher.