Suricata Rule Development Part 1

The stated offset value on this rule is 9, how do I figure out the minimum offset value that can be set to trigger an alert?

I tried changing this number randomly and running it over and over.

Guys, I found the answer by accident within your screenshot. Then, I tried to find logic in the rule, why the offset was set to that number, and many sources said different stuff. Can someone break down this better?