Something I forgot to mention in my previous post. The drain current Id is a function of the gate-source voltage Vg, namely Id=Id(Vg). Idss is defined as Id(0), and Vto is the voltage such that Id(Vto)=0. It is easy to measure these two parameters, Idss and Vto, if you have a device, but what you need for your device model is Beta and Vto, not Idss and Vto.
If you know the parameters Idss and Vto, you can express the drain current as Id=Idss*(1-(Vg/Vto))^2. Or if you know Beta and Vto, Id=Beta*(Vg-Vto)^2. Of course these two equations are equivalent, therefore you have Beta=Idss/(Vto^2), or Idss=Beta*(Vto)^2.
In the above example, the model is
.model 2SK192A NJF(beta=2.0m vto=-3.0 cgd=1p cgs=2p)
This model gives Idss=2.0m*(-3.0)^2=18m [A], as you can see from the figure.
And just one more thing..
A very simple design method for a source follower. You only need to decide three parameters, R2, R3 and R4. First, let R2=R3 so that the DC operating point is around Vdd/2(=6V). Next, you decide the Id. Looking at the Id=Id(Vg) curve, you choose Id=8mA and this gives Vg=-1.0V, which means R4=(6V-(-1V))/8mA~1kohm.
Since the source impedance is only 200ohm in this particular example, R2 and R3 could be anything, say 10kohm.
Finally, C1 should be small enough compared with R2//R3 at the operating frequency, so let C1 be 1nF, equivalent to 50ohm around 3.5MHz.