Question about Shockley's equation for JFETs
OK first a little on notation. Your Vgs(off) is also called the Threshold voltage (Vt) and the pinch-of voltage (Vp), I think they all mean the same thing.  The voltage where the device is off.
So I cracked open "Art of electronics" (2nd ed.),  They talk about two regions of operation.
A linear region where the the drain current is proportional to the gate source voltage.
Id ~  (Vgs-Vt) and a saturation region where the current is proportional to the square of that voltage.
Id ~ (Vgs-Vt)^2.  

The details are involved.  You might read some here.. and then look up the references.