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Regarding Sacred Geometry as a time-sink:
Since the feat increases casting time to a full round, I would rule that a player has time to work out an arithmetic solution until it is her turn again.
This work well since the result will be available when the casting is complete.
Regarding Calculating Mind:
(paraphrasing) "Any combination of d6s and d8s, not to exceed ranks in Knowledge(engineering)"
This seems to offer a lot more flexibility than the default Sacred Geometry!