Why would I change this percentage?

Reserved capacity is typically 40 percent of the base volume for snapshot operations and 20 percent of the base volume for asynchronous mirroring operations. Usually this capacity is sufficient. The capacity needed varies, depending on the frequency and size of I/O writes to the base volume and how long you intend to use the storage object's copy service operation.

In general, choose a larger percentage for reserved capacity if one or both of these conditions exist: