Creo Room is illegal. Creo Ind would create a single perfect mass. To get a perfect distribution, you will need Group. That would also solve the illegal Room Target. 10 cubic paces of flour should be enough to fill the room properly.
A level 4 CrIg does +5 damage, x4 for immersion gives +20. That makes your base 3 acceptable, but the secondary damage seems excessive for a pure Creo spell. If you would Rego natural flour into place or just CrAu, it would remove my "magic object have magically limited damage potential" objection.