MOSFET: What is considered "large voltage" when finding early voltage via experimental method?
I am not a specialist in Solid-State physics, but recall that the "Early Effect" was first identified for BJT, as a good approximation of I-V characteristics for different base currents. In MOSFET there is some resemblance (but versus Vgs), so the shapes of MOSFET I-V curves are also sometimes characterized by "Early Voltage". However, this is still an approximation, and it doesn't work well for MOSFETS. 

Identification of "linear I-V" section is somewhat subjective,  and the negative ("Early") voltages derived by extrapolation of I-V curves may not be identical for different Vgs curves, that's why you are having difficulties. 

In other words, the dynamic impedance (RdsON) for MOSFETs is not only dependent on Vgs, but also on Vds, that's why the RdsON is so fuzzy-specified in MOSFET datasheets.