Yet another MKMapView zoom level and Offline maps - ios

Question

I've been working with MKMapView in iOS 7 trying to set and get programatically the zoom level in order to download and reuse map tiles when I were offline.
As I can't download the whole map into my phone, I download just several tiles in an appropriated zoom level and, afterwards I fix that zoom level and use the tiles thought MKTileOverlay and MKTileOverlayRenderer.
I tried to use Troybrant's algorithm (http://troybrant.net/blog/2010/01/set-the-zoom-level-of-an-mkmapview) but it didn't worked well for me. It failed to establish the zoom level back properly.
So I created one of my own that works fine.
Some explanations about my own method:
At the maximum map zoom level (20), you would see every map point at 1:1 scale. The whole map would have 256*2^20 points.
In retina displays there is a 2.0 scale factor between map points and pixels.
Apple maps can vary zoom level from 3 to 19 (min & max)
Then there is a simple inverse rule:
At maximum zoom level our view will show as much points as pixels it has (with the point-pixel scale factor of 2.0 for retina displays)
If zoom level is decreased the amount of map points shown should increase (inverse rule)
With that information the idea is to set the MKMapView's visibleRect property:
visibleRect width points = 2.0 * mapView.bounds.size.width * 2^(20-zoom)
Using that formula I've been able to centre my maps and apply to them zoom levels properly.
As Troybrant did, I created a category with the following methods:
#interface MKMapView (ZoomLevel)
- (void)setCenterCoordinate:(CLLocationCoordinate2D)centerCoordinate
zoomLevel:(NSUInteger)zoomLevel
animated:(BOOL)animated;
#end
#implementation MKMapView (ZoomLevel)
- (void)setCenterCoordinate:(CLLocationCoordinate2D)centerCoordinate
zoomLevel:(NSUInteger)zoomLevel
animated:(BOOL)animated
{
MKMapPoint centrePoint = MKMapPointForCoordinate(centreCoord);
CGFloat rectWidth = 2.0 * self.bounds.size.width * pow(2, 20-zoomLevel);
CGFloat rectHeight = 2.0 * self.bounds.size.height * pow(2, 20-zoomLevel);
MKMapRect visibleRect = MKMapRectMake(centrePoint.x-rectWidth/2, centrePoint.y-rectHeight/2, rectWidth, rectHeight);
[self setVisibleMapRect:visibleRect animated:animated];
}
#end
I hope this code can help you all.