## coordinate of a point common to conic section and vector?

Hi,
I have the following problem,
I need to calculate the local radius of an ellipse at a given point. While I've got the formula to give me the radius at a Point (x,y):
R= (rr')^3/2/ab
where
r is the distance of the point of interest to the focal point c and r' to the 2nd focal point F'
the problem is here:

I calculate the point common to the ellipse and the vector by
q = atan(a*tan(Phi)/b)
P(x) = a*cos(q)
P(y) = a*sin(q)

Phi is the angle of the vector from the (0,0) origin of the coordinate system (the "center"of the ellipse)
I would like to have a way for specifying Phi as the angle from any given point on the minor axis rather than having to specify it from the origin (0,0)
Is there a way???
Help is much appreciated!
Thanks