Displaying a certain number of decimal places for doubles in the table widget should be as simple as using a delegate like:
Qt Code: 
  1. class Delegate: public QStyledItemDelegate {
  2.     Q_OBJECT
  3. public:
  4.     Delegate(QObject *p = 0): QStyledItemDelegate(p) { }
  5.  
  6.     QString displayText(const QVariant &value, const QLocale &locale) const
  7.     {
  8.         if (value.type() == QVariant::Double) {
  9.             return locale.toString(value.toDouble(), 'f', 2);
  10.         }
  11.         else
  12.             return QStyledItemDelegate::displayText(value, locale);
  13.     }
  14. };
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This is quite distinct from anything to do with what is displayed in an editor for that value.