Found this little gem inside a techdef
<GameModifier>
<Target>Player</Target>
<ModType>Resource</ModType>
<Attribute>DiplomaticCapital</Attribute>
<!-- Use the sovereign's charisma stat to determine how much diplomatic capital they get -->
<Calculate InternalName="Value" ValueOwner="PlayerSovereign">
<Expression><![CDATA[ 5 * [UnitStat_Charisma]]]></Expression>
</Calculate>
</GameModifier>
Since I don't really do spells, I just wanted to let people know about it. This came from inside a tech, so who knows if it works in spells (strategic or tactical, or tactical if sov isn't there).
I just did a very quick test and this appears to function (ie - high charisma sovereigns got much more benefit from these techs).
Oh, and I guess this means calculate can work outside a spelldef too.