Should the radius of the subsequent circles always be calculated from the center and then just double, triple, quadruple, etc the radius for the next circles...or should we be adding the radius offset to the radius + thickness. 

So for example if I have a radius of 10 and a thickness of 2 and all radii are measured from the center point should I be doing:

circle 1 radius = 10

circle 2 radius = 10 + 2 (the thickness) + 10 = 22

circle 3 radius = 22 + 2 + 10 = 34

OR should it be a straight even increase such as:

circle 1 radius = 10

circle 2 radius = 20

circle 3 radius = 30

despite what the thickness may be?

Its kinda hard to see, so I took her example and blew it up... each color is 1 pixel width so on her 3_20 there are 17 pixels between the first circle and the second...

I assume that means you ignore the thickness