You cannot rely on the value of UIWebView.CenterCoordinate after setting UIWebView.SetCenterCoordinate and I don't know of any event that is raised when it is finally set. To be more specific:

var map = new MKMapView();

var ctr = new CLLocationCoordinate2D(37.8, -122.4);
map.SetCenterCoordinate(ctr, false);
map.SetRegion(new MKCoordinateRegion(ctr, new MKCoordinateSpan(0.025, 0.025)), false);
Console.WriteLine("Center coordinate is still NaN: " + map.CenterCoordinate.Latitude.ToString());

Error: NaN Lat & Long var 
circle = MKCircle.Circle(map.CenterCoordinate, 100);

Must use explicit location instead, a la:
var circle = MKCircle.Circle(ctr, 100);


Lemme' get some SEO terms in here like MapKit, MKOverlay, and NaN