Here the actual code which is well reduced but still use the constant 20 :
Qt Code: 
  1. virtual QSize sizeHint( const QStyleOptionViewItem& option, const QModelIndex& index ) const override
  2. {
  3. 	// Unused.
  4. 	DE_UNUSED( index );
  5.  
  6. 	// Constants.
  7. 	const int RightMargin = 20;
  8. 	const QSize ItemPadding = QSize( 6, 6 );
  9. 	const int ItemWidth = option.decorationSize.width() + ItemPadding.width();
  10.  
  11. 	// Get the list widget using the parent value.
  12. 	QListWidget* ListWidget = static_cast< QListWidget* >( parent() );
  13.  
  14. 	// Compute the max width and the row item count.
  15. 	const int MaxWidth = ListWidget->viewport()->width() - RightMargin;
  16. 	const int RowItemCount = std::max( 1, MaxWidth / ItemWidth );
  17.  
  18. 	// Cases where the padding offset is not needed.
  19. 	if( ( ItemWidth >= MaxWidth ) || ( RowItemCount >= ListWidget->count() ) )
  20. 		return option.decorationSize + QSize( 0, option.fontMetrics.height() ) + ItemPadding;
  21.  
  22. 	// Compute the padding offset and add in the item size to compensate of empty space.
  23. 	const int PaddingOffset = ( MaxWidth - ( RowItemCount * ItemWidth ) ) / RowItemCount;
  24. 	return option.decorationSize + QSize( PaddingOffset, option.fontMetrics.height() ) + ItemPadding;
  25. }
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If a way to have it really better and still working good exists, i'm open to listen how solve that.