How to Estimate Fet V Cut Off
I am really over my head with all of the semiconductor terms and math but here goes. 
I have found several references that state that sub-threshold Ids drops by a factor of 10 for every n*60mV drop in Vgs below V threshold.  Where "n" is a characteristic of the FET:
n = 1 + Cb/Cg ; Cb is bulk capacitance, Cg is gate capacitance
I have not yet found a rule of thumb for maximum "n" but it seems that maybe; 1.0 &lt; n &lt; 1.4

Rule of Thumb: Perhaps I could assume that for every 100mV below the threshold voltage, Ids will decrease by 10x.  So if the data sheet specifies (Vth = -0.45, Ids=-250uA) If I get |Vgs| &lt; |-0.21| then |Ids| &lt; |-1uA| 

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