Hmm... wouldn't be better to have element, demi human and matk modifier instead?
I find it better to balance matk output for all out situations instead of just 1 element spell. Warlocks are versatile so they should be able to manage more than 1 spell.
If I remember correctly, my current set up gives me +105% demi human, +60% matk, +50% wind magic damage and 77% mdef piercing.
I find it to be a good balance overall damage wise.