Distance Calculation - from Isometric view

Question

I need a lot of help with this one. I uploaded an image so
you can see what I am talking about because I know this will help
explain what I am trying to do.
I made an isometric world and am trying to calculate the
distance from the center to the outter edge of the circle (or map).
The red dot in the middle is the middle. Looking at the image its
obvious that the green line is longer than the blue line, yet
BECAUSE its an isometric world, the distance is exactly the same.
Right? So here is the real problem: How do I calculate, or
determine, the distance of the orange dots from within red dot in
the middle? I know it isn't using the a^2 + b^2 = c^2 method -
because that doesn't relate to isometric stuff. My guess is some
how using angles, but I have had no luck :(.
What do you guys suggest, and can you provide some small
snippets to help me? Many thank yous
*Here is the example picture:
http://www.gmajr.com/example.jpg

Answer 1

nice :) (i knew you'd hit this one, lol)
here's my 'limp' animated demo version, using a couple of
known ratios of the conic plane, in relation to an actual circular
region:
FLA
...does it, but not very elegant ;)