Basic functions for calculating points around a circle based on angle. (r = radius of circle)
x=r*cos(a)
y=r*sin(a)
So first well take the function for finding x, based on angle, and invert it to find the angle based on x.
x=r*cos(a)
x/r=cos(a)
arccos(x/r)=a
Now stick that into the function for finding y as the value for angle.
y=r*sin(arccos(x/r))
The x, and y values you use should assume that the center of the circle is at 0, 0. If not subtract the centers x from your x value before calculating, and add the centers y to the result. You'll probably also have to make the result negative first. It's been a while since I've actually used these calculations, so I apologize in advance if I got something wrong.