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QPainter drawArc question

Hi. I'm trying to use QPainter drawArc(const QRectF & rectangle, int startAngle, int spanAngle) for arc drawing.
And have a problem with it.
I have a coordinates of 3 arc points A, B, C.
A  arc start point
B  arc end point
C  arc center with known radius (10).Question. How to calculate rectangle, startAngle and spanAngle for this arc?
Look Arc Image

NOT TESTED, JUST MATH (and i hope it is correct ^^)
eg.: Ax  x coordinate of A point, atan === ArcTan, abs === AbsoluteValue
Rect Top Left Point: (Ax,By)
Rect Bottom Right Point: (Cx+R,Cy+R)
...or minus im not sure about the coordinate system inside drawing but i think youll get the point i hope
Remember 0 Degree is at 3 o'clock so:
Angle to B point:
atan( abs(ByCy) / abs(BxCx) ) * 16 (it should be in 1/16th Degree thats why *16)
Angle to A Point:
(90  atan( abs(CxAx) / abs(AyCy))) * 16 (90 couse 0 is at 3 o'clock, "" because clockwise)

Arc can be located in any of the quarters. I'm drawing using coordinates from CNC file (GCode).