LHC ultimate party setup.

I believe in normal training locations it's +10% exp for a party of 2, +5% exp for every member after. So you'd get +30% exp for a party of 6 at ToT. The 50% stacks additively with that.

Not entirely sure if that's correct.

That is correct.
Normal party bonus is 5% per party member, including the first (hence, 10% for a party of 2, 30% for 6).
LHC party bonus is added on top of that, and is 50% for every *extra* member, so doesn't include the first. Hence, 280% for a full party.

A non-leeching mule doesn't add to party bonus. Just checked it to make sure. So HSer needs to be leeching.