Generally, credits are taken for the portion of the already billed period that is being canceled.
For example, if at 10 days into March, a service is canceled, and assuming it was billed through the end of March already (on a monthly frequency), Manitou credits 21 out of 31 days of the total invoice. It is the remaining days (not including the day of the change) divided by the total days of that specific period that will be credited.
Rate tables have no bearing on credits, as they are used to calculate charges. Actual billed days are used to calculate credits.