This is a math question, I like :) .
The trick is to find the distance between center and lower right corner.
Then using Cosinus and Sinus the transformation of X and Y position can be calculated.
Sinus transforms an angle (in radians) to a floating number between 1 (90 degrees) and -1 (270 degrees).
Cosinus does the same but is 1 at 0 degrees and -1 at 180 degrees.
A possible mistake may lie in calculating 'Radius' though I believe I did it pretty well.
float defaultAngle = 45;
PointF center = new PointF(width/2,height/2);
float Radius = Math.Sqrt(Math.Pow(width - Center.X,2) + Math.Pow(height - Center.Y));
fullAngle = defaultAngle + yourAngle;
PointF BottomRightCorner = new PointF(0,0);
BottomRightCorner.X = center.X + Math.Cos(Radians(fullAngle)) * Radius;
BottomRightCorner.Y = center.Y + Math.Sin(Radians(fullAngle)) * Radius;
float Radians(float Degrees)
return Degrees * (Math.PI /180);