_calcRangeOffset() {
      let width = this.$el.width() - 15;
      let max = parseFloat(this.$el.attr('max')) || 100; // Range default max
      let min = parseFloat(this.$el.attr('min')) || 0; // Range default min
      let percent = (parseFloat(this.$el.val()) - min) / (max - min);
      return percent * width;
    }
Calculate the offset of the thumb
@return {Number}  offset in pixels