On RedHat it can be set in /etc/sysconfig/ntpd but despite much searching on web and also reading man page for ntpd, I cannot find how you do this on Ubuntu (set -x option on ntpd).  I can manually kill ntpd on Ubuntu and then set it by re-launching ntpd manually, i.e.sudo /usr/sbin/ntpd -p /var/run/ntpd.pid -g -x -u 128:138but I'd like these settings to take effect at startup on Ubuntu.
Set the -x slewing option for NTP startup
ubuntu;ntp