I know who made the chart... Me.
I'll be the first to say it probably isn't as accurate as possible. But then again, I don't have the time/money/equipment to test all the springs out there... the chart is formed as much as possible from information that is readily avaliable (i.e. manufactuers numbers), otherwise it is what I've been sent/read with regard to other peoples findings.
The biggest problem is Tamiya don't release the rates of their springs (believe me, I've tried to get it before)... and without knowing the Kc (spring constant) value of the material, and lacking proper load reading equipment to do it myself, the values are only as accurate as I have been told.
I am going to look into getting my Tamiya springs tested this week, might be able to pull some strings at work over it...
Ed