Compared to the open lock skill, this spell can open only low DC locks. The formula should be redesigned to give us a chance to improve it.
Open Lock: 28 ranks +X dexterity + 1d20
Knock spell: 25 CL + X ability + 1d20
Open lock of course gets a TON of items/abilities to increase it:
+7 tools + 20 competence +5 enhancement +6 greensteel +4 morale +2 luck +5 action boost +6 epic twist, plus a bunch of others
* Base it on the Spellcraft skill?
* Let the spell/dragonmark consume tools as optional component?
Possibly something convoluted like:
spellcraft base ranks (or character level for dragonmark) + int/cha + 1d20 + competence bonus to open lock item + enh.bonus to open lock item +6 int/cha skills item +4 morale +2 luck +5 action boost +6 epic twist
Basically, make it a skill check so that we can use all the skill increasing bonuses.