Suppose a man is standing at position xCurrentPt, yCurrentPt and with angle "Angle". Now he want to move to position xTargetPt, yTargetPt. Before walking he must rotate himself to a proper angle value. He'll continue to walk with his speed 'fSpeed' until he's reached the Target position (xTargetPt, yTargetPt)
The problem is I can't get the new right angle when the target has been changed.
Now all you need to do is to calculate the angle between those two lines (think of it as a triangle, which means, connect the end of the horizontalLine with the end of the verticalLine). It probably helps to make a sketch.