In feat.2da inv_eldrtchdoom is referencing a TGA file. The impact script for Eldritch Doom (spell.2da #18142) is inv_eldtch_shape.
Ha, yeah, that's the icon. -_-
Was getting flustered at that point.
The Blast DC is calculated in inv_eldritch_shape.nss on line 62
int nDC = 10 + nBlastLvl + GetAbilityModifier(ABILITY_CHARISMA);
Hope that helps you out. 
Hmm... Due to the section heading, I thought the references to blast shape level in there were solely for the calculation of the DC of the essence effect. I suppose since both are keyed off of the greater of the essence and blast shape level, and the outcome should always be equal in the absence of some obscure effect, it would be silly to make a separate section.
Ok, thanks!