public enum Quadrants : int { nw = 2, ne = 1, sw = 4, se = 3 }
private double GetAngle(Point touchPoint, Size circleSize)
{
    var _X = touchPoint.X - (circleSize.Width / 2d);
    var _Y = circleSize.Height - touchPoint.Y - (circleSize.Height / 2d);
    var _Hypot = Math.Sqrt(_X * _X + _Y * _Y);
    var _Value = Math.Asin(_Y / _Hypot) * 180 / Math.PI;
    var _Quadrant = (_X >= 0) ?
        (_Y >= 0) ? Quadrants.ne : Quadrants.se :
        (_Y >= 0) ? Quadrants.nw : Quadrants.sw;
    switch (_Quadrant)
    {
        case Quadrants.ne: _Value = 090 - _Value; break;
        case Quadrants.nw: _Value = 270 + _Value; break;
        case Quadrants.se: _Value = 090 - _Value; break;
        case Quadrants.sw: _Value = 270 + _Value; break;
    }
    return _Value;
}