I always put it to my email. I save it in my drafts and applying some secret code to the phrase just like adding some letters on it so that if ever my email was hack. They will not use it since it was wrong and it is very hard to determine what is the character that it makes wrong. Trust me, No one can use it but just remember what and where do you put the additional character.
I'm afraid your way is not good in case you forgot the rules set by yourself
I am completely satisfied with this way, I can share with you my way is:
1. I write on paper and store with my most important documents, all stored in a safe
2, Printed and stored in the cabinet
no one can steal yours