IMO this spell doesn´t need add the +1 magnitude for complexity/enhance the effect. I prefer to consider the Dust to Dust effect more like a cosmetic effect more than a real benefit in gameplay.
Surely is arguable that if the body is destroyed necromancers can´t raise it, but magic is very rare in Mithical Europe and probably you don´t need to worry about that possibility. At least no more than other minor benefits anyone can take from this cosmetic effects ( Imagine the mighty torrent of water from the example of Marbaid, he makes a Mighty torrent of blood instead anda the blood persiste until sunrise/sunset, he doesn´t add magnitudes for requisites or complexity, but i can imagine a few of possibilities when this can be very useful).
But anyway, if its only a cosmetic effect, unlike dust to dust, this spell it´s unable for killing mindless undeads like the original dust to dust spell.
Excuse me for mi english, i know it is very bad, but i hope we can undesrstand me.