I don't think anyone knows the formula 100% for sure, though i've definitely seen a couple of posts where people explained how it works for the most part.  If you search you should be able to find something.

There is this section in ElementalDefs.xml if you're looking to mod it globally:

<!--** LEVEL UP **-->
<!-- ** combat rating -> experience control ** -->
<CombatRatingToExpMultiplier>0.45</CombatRatingToExpMultiplier>
<!-- <StatPointsPerLevelUp>3</StatPointsPerLevelUp> (Obsolete) -->
<MaxExpFromCombat>-1</MaxExpFromCombat> <!-- set to -1 to disable -->
<MinExpFromCombat>1</MinExpFromCombat>
<MultiplierOnPointsToLevelUnit>1</MultiplierOnPointsToLevelUnit>
<ExpBaseOnPointsToLevelUnit>2.1</ExpBaseOnPointsToLevelUnit>
<ExpMultiplierOnDesignedUnits>0.5</ExpMultiplierOnDesignedUnits>
<UnitLevelUpPerkOfferCount>5</UnitLevelUpPerkOfferCount>
<CityLevelUpPerkOfferCount>3</CityLevelUpPerkOfferCount>
<!-- when trained units get XP, they use it towards leveling up, but also give this percent to the sov. -->
<XPPercentTrainedUnitsGiveSov>0</XPPercentTrainedUnitsGiveSov>

I could be wrong, but I don't think you have to do that anymore... in D&W I add some entries to ElementalDefs.xml using a small file in the data folder... it just has the new entries.

I vaguely recall that somewhere along the ling they changed it so you don't have to duplicate the whole file, you can just put changed entries in the mod folder.

Mate i'm not sure what I was on about there... because I've just been looking and it doesn't look like i'm touching the ElementalDefs.xml.  I have not idea, at this stage i'm going to assuming it was the post of a drunkard.... but i'm going to look a bit further and see if I had something there at some point.