What instance can I use my 80s without gimping exp?

I'm sure this question has been asked before... so, sorry :)

My two lowbie moonkins are now 68 and ready for Northrend. I would like to use my 3 80 druids to level them through instances, however I'm not sure the level/instance that the experience penalty goes away. Anyone know? Am I thinking about the experience gimping correctly?

In other words, I'm assuming the experience for two level 77s would be the same in a level 78 instance even if I had 3 80s as compared to 3 77s. What is the lowest level instance where this dynamic holds true? I thought I remember reading if the mob is 9 levels below an 80 (ie level 71) then it is still green to the 80 and everyone in the group will get full exp. However, if the mob is more than 9 levels (70 and below) then the exp will be gimped.

Your thinking of the gimping wrong. You'll be gimped with 3 80s in your group no matter where you go. The XP allocated to each toon will be reduced by a consistent portion no mattuer what mob your killing. The total xp returned BEFORE gimping is determine by the level of hte mob.

Optimally, you should only have one 80, and your lowbies. You would be best off finding other players to fill out the group that are the same level or lower than your lowest level toon. Any instance with mobs 1-5 levels higher than your lowbies should yield the best results.

for doing an instance...
I would guess that the targets need to be valid to both the lowbie and the 80.
if it's too low to the high charcter is not good, if it's too high for the low character it's not good.
the min valid level for the 80 character is 72.
So, if 72 isn't too much higher than your 68, it should give some pretty good points.
Drak'tharon or maybe the old kingdom instances might work.

You could probably put the two lowbies in a team together, and follow them with the high characters and kill mobs over 72 in dragonblight for good rested XP.