drawArc in the rectangle issues

[doris]

drawArc.jpgwanted.JPGwanted.JPG

my program will get the startpoints(x,y) and calculate width and height when you draw on the scene, the attached photo shows it.
the problem is i want to draw a perfect arc in the middle of rectangle as the sketch one attached.
my code as below:

void ResizableRectMarker::paintArc1(int x, int y, int width, int height, QPainter* painter)
{
  double radius = sqrt(pow(0.5*width,2)+pow(0.5*height,2));
  double step = sqrt(pow(radius,2)-pow(0.5*width,2));
 
  QPointF topLeft(x,y);
  QPointF bottomRight(x+width,y+height);
  QRectF rect(topLeft, bottomRight);
 
  int startAngle = atan2(0.5*width,step)* 180 / M_PI * 16;
  int spanAngle = 2*startAngle;
  qDebug() << "start angle:" << startAngle << "|" << "spanAngle:" << spanAngle;
  painter->drawArc(rect, startAngle, spanAngle);
}

void ResizableRectMarker::paintArc1(int x, int y, int width, int height, QPainter* painter)
{
  double radius = sqrt(pow(0.5*width,2)+pow(0.5*height,2));
  double step = sqrt(pow(radius,2)-pow(0.5*width,2));

  QPointF topLeft(x,y);
  QPointF bottomRight(x+width,y+height);
  QRectF rect(topLeft, bottomRight);

  int startAngle = atan2(0.5*width,step)* 180 / M_PI * 16;
  int spanAngle = 2*startAngle;
  qDebug() << "start angle:" << startAngle << "|" << "spanAngle:" << spanAngle;
  painter->drawArc(rect, startAngle, spanAngle);
}

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the problem is the the arc drawn is not in the center, and also if the rectangle is not a square, the arc is wired.
any solution for it.

newbie here..