Added twist to a script - Math needed eek

I’m not a math guy so I’m having a real hard time with a script I’m working on.

Here is my math problem that needs an equation
I have script where I enter the longest side of the image I want.
This script then makes rounded corners based on a whole number.
I have found that for following sizes based on 100, the corresponding numbers looks best.
100, 2
200, 5
400, 8
600, 9

I don’t think this is an exponential scale, but I need an equation in which whatever number I put in for 100, 200, 400, etc. I get the corresponding number.
Checking with mathforum.org I have found the term I might trying to perform is called regression.
http://mathforum.org/library/drmath/view/62168.html

The equation they provided I think is y = mx + b
I believe my imput (100, 200, 300, etc) is the variable x, but have no idea how to go from there.