Carpenter's square mod

I needed to know some tube diameter and was struggling to take correct measure. I don't have a typical caliper at home so I used carpenter's square and ruler. Then I thought I could design this little upgrade and made myself a caliper.

This design should fit any of this classic yellow squares (4cm wide). I'm uploading two versions, one is more tight than other. I used the standard version and its just fine.
Feel free to comment if you find different dimensions.

This design is still work in progress and I have an idea for more sophisticated version. Hopefully coming soon. I will appreciate if you share your thought and ideas too.