Responsive layout using flexbox and grid, javascript calculations

I had some issues with trying to figure out how to deal with the percentage coming in from a button and also a number input. I know there must be a more straightforward way. Looking forward to seeing what others did to handle this.

Also, I couldn't figure out why when I tried to move an inline event handler I had on a parent container to my app.js why it stopped detecting a child input. I left it inline but curious if anyone might know why this would be. The only thing I could think was that for some reason the clickhandler needed to be attached before the app.js file was loaded?