Size of scatter markers

I want to calculate the intersection of the edge of a marker with a line
running from the center to another marker. I want to do this in order to
decorate the intersection.

I'm using the following code(derived from some code in to try
and "divine" the size:


This is not right. Clearly, I just don't have an idea what I'm doing
(when it comes to calculating the distance along a vector(the along a
vector part is easy) of the size of scatter marker). Anyway, who can
point me in the right direction would be my hero for atleast 2 or 3