Andrew, I don't know if this is any better but here is another way: private function calculateDistanceFromPixels(tolerance:Number):Number
{
var screenPoint:Point = map.toScreen(map.extent.center);
var upperLeftScreenPoint:Point = new Point(screenPoint.x - tolerance, screenPoint.y - tolerance);
var lowerRightScreenPoint:Point = new Point(screenPoint.x + tolerance, screenPoint.y + tolerance);
var upperLeftMapPoint:MapPoint = map.toMap(upperLeftScreenPoint);
var lowerRightMapPoint:MapPoint = map.toMap(lowerRightScreenPoint);
var ext:Extent = new Extent(upperLeftMapPoint.x, upperLeftMapPoint.y, lowerRightMapPoint.x, lowerRightMapPoint.y, map.spatialReference);
return ext.width;
}