Google Maps - Deriving Lat/Lon points - latitude-longitude

Question

I would like to derive the lat/lon which stands x inches east and y inches south from a given lat/lon point.
For this purpose I used information and formulas from the following link: http://www.appelsiini.net/2008/11/introduction-to-marker-clustering-with-google-maps
Basically my methods do the following:
public static double adjustLongitudeByPixels(double longitude, long pixelsY) {
int zoom = 21; // use zoom level 21
double scaledPixels = (pixelsY << (21 - zoom)) / GOOGLE_SCALING_AT_ZOOM_21;
return xToLongitude(longitudeToX(longitude) + scaledPixels);
}
public static double adjustLatitudeByPixels(int overviewScale, double latitude, long pixelsX) {
int zoom = 21; // use zoom level 21
double scaledPixels = (pixelsX << (21 - zoom)) / GOOGLE_SCALING_AT_ZOOM_21;
return yToLatitude(latitudeToY(latitude) + scaledPixels);
}
...where longitude/latitude are the ones from the starting point and the pixels are the number of pixels east or south from the starting point.
I obtain the pixelsX and pixelsY by multiplying my real x/y inch distances by 96 (number of pixels per inch). So I basically obtain the pixels at scaling 1:1.
Since the latitudeToY and longitudeToX use units pixels at zoom level 21, I divide by the scaling at zoom level 21.
If I am not wrong this scaling is 1:282.124305.
This method does not yield the expected results, though. The resulting lat/lon are slightly different than what I expect.
Please let me know if I have flaws in my logic...
Thanks in advance!