Line regulation of zener diode with jfet
The FET is being used as a constant current source. 
Reference to the datasheet shows the current could be anywhere from 6-13 mA.

Your should read this Siliconix/Vishay app note to understand the elements around setting up a CC source with a JFET. Providing the FET is in saturation (V(DSS) is high enough) you can assume that the conductance is approaching zero. 

Your deltaVout/DeltaVin will approach zero providing the FET is in saturation.