JFET Q-point for use it as resistor
Many SPICE simulators are good for exploring such arrangements. Learn to exercise their plotting capabilities. Here's a LTSpice run of a common N-channel JFET that came up first (don't take this as a recommendation for use in this application).

The test circuit applies a small 0.5V(peak) 1000 Hz. sine wave voltage to the drain-to-source points, where the "variable resistance" is of interest. Gate-to-source voltage is stepped through a list of DC voltages.
Current vs. voltage is plotted. An excellent result would be a set of straight lines of various slopes:

For Vgs=0V, this JFET looks like an almost linear resistor of about 138 ohms.
For Vgs=-1.0V, linearity is visibly deteriorating. It is changing between about 175 - 222 ohms. For more negative gate voltages, linearity becomes worse. At these larger gate voltages, the FET is useful as a linear resistor only for very small drain-to-source AC voltages.