get _progressPercentage() {
    if (this.indeterminate) {
      return undefined;
    }
    return ((this.value - this.min) / (this.max - this.min)) * 100;
  }
In case of a determinate progress-indicator it returns the progress percentage
based on value, min & max.
Could be used for styling inside the _graphicTemplate

@example
style="width: ${this._progressPercentage}%"