NTP Server and Client millisecond precision sync

Question

I'm facing an issue with getting my NTP Client (Running on Ubuntu) to have it's time sync to a NTP Server (running on windows) to millisecond precision.

The NTP Server runs using an open source python module: https://github.com/limifly/ntpserver

The NTP Client has server 192.168.0.28 prefer iburst in the ntp.conf file.

I found that the client requests for the time every 2 - 15 seconds.

To check the time I have a simple python script running on both devices and I'll do a screen shot with both print outs displaying:

from datetime import datetime

def main():
    while True:
        try:
            print datetime.utcnow().strftime("%Y-%m-%d_%H-%M-%S.%f")
        except KeyboardInterrupt:
            print "Exiting..."
            break

if __name__ == '__main__':
    main()

I am getting deviations of 5 seconds to 100 milliseconds.

When I run the command:

sudo service ntp stop
sudo ntpdate -s [NTP SERVER ADDRESS]
sudo service ntp start

I found that I can get 70 milliseconds to 5 microseconds precision immediately. After about 5 minutes I can see that the time deviates by 7 seconds...

My goal is to get millisecond precision so my question is am I doing everything correctly to achieve this goal? Also is there a better way to measure the time sync time?