You want to improve on the default value in the ntp clock-period xxxxxx command that automatically appears when you configure NTP on your router.
The router automatically generates a ntp clock-period line that it uses to help speed resynchronization after a reboot. Leave it alone:
Router#show running-config | include clock-period ntp clock-period 17180200 Router#
Do not attempt to remove or modify the ntp clock-period command. The router automatically generates this command to compensate for internal timer inaccuracies.
Do not modify or remove this statement from the router configuration. The router automatically adds this statement to the configuration when you enable NTP. After initialization of NTP, the router tunes the clock period to compensate for precision problems with the router's internal clock.
The router will actually constantly change the NTP clock period. For this reason, Cisco suggests that all routers have their configurations saved to NVRAM one week after enabling NTP. This ensures that the saved clock period is accurate for the next router initialization and allows for quicker NTP synchronization.