If you need some more specific CSS you need to put it into quotes - react inline styles doc


In your exact case

customFormat = 'hello-div'
divStyle = {width: 'calc(100% - 276px)'}
return (
    

      Hello World
    
)
In case you need to overwrite multiple widths (fallbacks) for browser compatibility

divStyle = {width: 'calc(100% - 276px)',
    fallbacks: [
        { width: '-moz-calc(100% - 276px)' },
        { width: '-webkit-calc(100% - 276px)' },
        { width: '-o-calc(100% - 276px)' }
]}