This was for 95 cap, 99 might squeeze it down a tad more:
Edited, Feb 7th 2013 5:36pm by Raelix
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You wanna be doing page 4 on the third map.
The problem starts when you get a level 90+ in the run for a trial or whatever. I do know a 95 can still PL in duo a level 8 though, I think they get 2 exp from Leeches; just can't have anybody else in the group.
Exp is normally weighted per player by the total exp needed for their level, but if you get more than seven levels below it just goes to your level divided by theirs. This is actually advantageous because 1/95 is larger than 500/54,000 (larger as in you get more exp).
Just getting five or ten levels outside of PLing means you only need, after correcting for shares in duo with signet, 14-26xp/kill before distribution instead of 95xp/kill, which works out to 19-35xp/kill solo at 95. Being in a full alliance multiplies this by ten though (1.8x total share divided by 18 members), so you'd have to get to
level 50(close enough, see below) to be getting 1xp again if you joined a full alliance with any level 95s!
So in short, I think you can duo PL a level 10 on anything you'd get at least 19xp from at 95 and they'll get 1xp/kill. Interestingly the lowest recorded EP exp return is 20xp (a level 58 mob, though I believe level 56 is still EP), so it seems being at least level 10 means you can jump into almost any GoV at all and get at least 1xp so long as you stay duo (there may be an exception for exactly level 56 or 57 mobs, which would give 16xp and 18xp respectively just looking at the pattern, but those practically don't exist in GoV).
And indeed, the lowest level bloodsucker in Bost*tab* (don't look at the 'fished up' entry!) is level 65. 34xp/kill to 95 solo, the minimum level to get exp in duo with a 95 is level 4:
( 4/95 * 0.75 * 34 ) = 1.07
and a little back-calculation to be sure, for the minimum level for the lowest possible EP in duo with 95:
( level/95 * 0.75 * 16xp ) >= 1
Comes out to needing to be level 8. There's your pre-leveling targets if you want to GoV grind in duo with a level 95. 8 gets you anywhere, but you can go to Oubliette leeches at 4!
If you stuck to a single party of six with a 95 on leeches, the lowest you could be to get 1xp from leeches with any lv95s is:
( level/95 * 0.35 * 34 ) >= 1
Also level 8. So that's what I'd recommend if you want to prime a job for GoV grinding, since you can fill your party with anybody else that may want to tag along.
So to double check since I changed methods and I doubt a figure earlier in this post, a full alliance with a 95, Oubliette leeches again:
( level/95 * 0.1 * 34 ) >=1
Comes out to level 28.
Full alliance, lowest possible EP, w/ 95:
( level/95 * 0.1 * 16 ) >=1
You can see how disadvantageous a GoV alliance is to very, very low level players, but if you stick to single parties you only need to be level 8.
These are all WITH SIGNET! I kinda glossed that over. It's very important for boosting your share when in duo. The lower level player has to have it!
t;dr you can get exp in a single party with 95s at level 8, in exclusively duo on oubliette leeches at level 4, and in any alliance on oubliette leeches at level 28. YMMV in partial alliances, but they are always disadvantaged to a single party.
Fynlar brought up a good idea, but if you pop an anniversary ring for double exp it still only directly halves the level you need to be, so at leeches you still have to be level 2 in an exclusive duo with a 95, 4 in a party, but now you can join a full alliance at 14!