How do I make an Overlay display a variable?

I was creating a mod that gets the amount of items in a backpack of sorts and displays that in an overlay.

I was wondering how to make this value variable.

Currently, I have it to where the text uses: "ENBT:number:tagname", as it sets that tag to a variable storing the value of the items. I don't know if this is incorrect, or if I am just missing something.
 

Yes, I have already looked at this link: https://mcreator.net/forum/49725/variables-and-overlays-how-tutorial , but I prefer to not work in java as much as I can.
Thanks for help in advance!