LHC ultimate party setup.

I kept reading that the party bonus is 50% per party member but that doesn't seem to be right.

Party of 6 with HS at Bearwolves -> 52486 exp + 146905 (party exp)
Party exp is 280%.
Party of 5 (with HS, didn't kill it all) -> 32934 + 74102
Party exp is 225%.
Party of 4 -> 37420 + 63614
Party exp is 170%.

I'd heard it was +50% per party member, but it's actually 55%? Does it drop down to 115% for 3, and 60% for 2?

Also I'll note that HS does seem to give the full 50% exp bonus, despite what I read somewhere else.