Grid cell size calculator.

new.m

// Get the view's actual pixel width
CGFloat pixelWidth = self.collectionView.frame.size.width * UIScreen.mainScreen.scale;

// Maximum cells per row
CGFloat maxCells = ( UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad ) ? 10.0 : 5.0;

// Determine a cell width
CGFloat cellWidth = 0.0;
CGFloat cellSpacing = 2.0;
for( CGFloat i = cellSpacing; i <= 4; i++ ) {
    // Reset the cells per row
    CGFloat cellsPerRow = 4.0;
    // Find an appropriate number of cells per row
    while( fmodf( ( pixelWidth - ( i * ( cellsPerRow - 1 ) ) ), cellsPerRow ) != 0 ) {
        // Iterate the number of cells
        cellsPerRow++;
        // Break out if it gets stupid
        if( cellsPerRow > maxCells ) {
            break;
        }
    }
    // Eureka! We've got it!
    if( cellsPerRow < maxCells ) {
        cellWidth = ( ( pixelWidth - ( i * ( cellsPerRow - 1 ) ) ) / cellsPerRow ) / UIScreen.mainScreen.scale;
        cellSpacing = i / UIScreen.mainScreen.scale;
        break;
    }
}

// Fix this size business
self.collectionViewLayout.itemSize = CGSizeMake( cellWidth, cellWidth );
self.collectionViewLayout.minimumLineSpacing = cellSpacing;
self.collectionViewLayout.minimumInteritemSpacing = cellSpacing;

original.m

// Determine the correct size
CGFloat size;
if( UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad ) {
    size = UIInterfaceOrientationIsLandscape( [[UIApplication sharedApplication] statusBarOrientation] ) ? 169 : 152;
}
else {
    size = ( UIScreen.mainScreen.scale == 2.0 ) ? 78.5 : 78;
}

// Apply the size and cell spacing
CGFloat max = floorf( self.view.frame.size.width / size );
self.collectionViewLayout.itemSize = CGSizeMake( size, size );
self.collectionViewLayout.minimumLineSpacing = ( ( self.view.frame.size.width - ( max * size ) ) / ( max - 1 ) );
self.collectionViewLayout.minimumInteritemSpacing = ( ( self.view.frame.size.width - ( max * size ) ) / ( max - 1 ) );

The original code was designed to output 4 cells on iPhone/iPod touch, 5 cells on a portrait iPad, and 6 cells on a landscape iPad.

The new code replicates the results, but isn't tied to a specific screen size.