Simple N-JFET circuit with PWM
I think I got it.

Since I established the JFET works in the ohmic region, its resistance \$R_{DS}\$ can be approximated by assuming that \$I_D(V_{GS})\$ is linear in this region and so \$R_{DS} \approx \frac{|Vp|}{I_{DSS}} = 400\Omega\$.

The equivalent resistance of this circuit will be \$R_D + R_{DS} = 10k\Omega + 400\Omega = 10.4k\Omega\$ and the drain current \$I_D = \frac{V_{DD}}{R_D + R_{DS}} = \frac{10V}{10.4k\Omega} \approx 961.54\mu A\$. There is a difference of less than two percents between this current and the one given by the simulation, leading me to conclude this should be correct.

Thanks Peter Smith for pointing me to the definition of \$ I_{DSS} \$.