Sass/css - width from browser edge to grid system column

SassMeister-input.scss

// Calculates the width of something that hugs the browser edge 
// to a column inside a grid system with a max-width (Tachyons)

.sideimg {
  $mw: 1920px; // max-width of container
  $col: 40; // tach col width in percentage
  
  // this doesn't evaluate
  width: calc( 50% + ( -$mw / 2 + $mw * $col/100 ) );
  
  // simplify using #{}
  width: calc(50% - #{$mw / 2 - $mw * $col/100});
}

SassMeister-output.css

.sideimg {
  width: calc( 50% + ( -$mw / 2 + $mw * $col/100 ) );
  width: calc(50% - 192px);
}