I wouldn't doubt that there is no standard for such a measurement. One issues is that springs are not entirely linear, meaning the spring rate can vary with deflection. However, once preloaded to the extent that most springs are in actual use much of the initial deflection non linearity is no longer present. All this aside one number, the "rate" is typically well suited for a characterization of the spring and it should be reported.
Do you think race teams know the exact spring rates they install in their cars?
Sounds a bit lame on Eibach's behalf...