You can increase reserved capacity only in increments of 4 GiB.
Review the following guidelines:
If no free capacity exists on any pool or volume group, you can add unassigned capacity in the form of unused drives to a pool or volume group.
For asynchronous mirroring operations reserved capacity is typically 20 percent of the base volume. Use a higher percentage if you believe the base volume will undergo many changes or if the estimated life expectancy of a storage object's copy service operation will be very long.