What it appears you are trying to do (can’t really know unless you actually ask a question with words) is set the progress bar to dynamically change based off your bosses health? You’ve got part of it right but you need to “Bind” the progress bars event. Select the progress bar inside the designer, next to it’s value click “Bind” and select “Create Binding”
In the “Get Percent” that you already have setup, simply save that to another variable called “BossHealth” or something, the simply plug the variable “BossHealth” into your new binding function.
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Also worth noting that in the image you showed of your “Get Percent” function, the Cast is wrong. You can’t simply put “dust_boss” as the object - it won’t work. You need the instance of the boss in the world. What you ask for in it’s current setup is for the BP which does not exist because it’s not instantiated into the world. Get your instantiated dust_boss reference and pass that into the Cast node.