I've been playing with this circuit:

It's simple tuned FET amplifier. It's not really practical circuit, it's just an example to see if I can calculate how to bias a FET.
Let's begin by writing what I think I know:
Tank circuit is short circuit for DC. Therefore, for calculating bias, drain voltage is 12V
Id=Vcc/R1=4mA
We will choose quiescent point at Id/2 - 2mA.
By using these methods http://www.diyaudio.com/forums/pass-labs/228897-jfet-matching-sorting.html
I measured for my JFET BF244A:
Idss=5.43mA which is about right looking at the datasheet.
Vgs,off=2.19V - again, about right
Now I have to calculate what Vgs I need to have 2mA drain current.
Using upper equation. When I turn it around I can calculate Vgs and I get Vgs=0.86V
That means Vg=Vs+Vgs=6+0.86=6.86V
From that I calculate that in voltage divider I have values from the schematics. But in real life it's not like that. I get right gate voltage, but source voltage is always about 2V off from my calculations. I understand that I my measurement of Idss and Vgs,off may be wrong, but I don't have another way of measuring them.
I tried having 4V at source and then doing the calculations and in practical circuit I got around 6V. If anything, it's always about 2V and I can just do the calculations like that, but I would prefer to know what is happening here.
What am I doing wrong here?

It's simple tuned FET amplifier. It's not really practical circuit, it's just an example to see if I can calculate how to bias a FET.
Let's begin by writing what I think I know:
Tank circuit is short circuit for DC. Therefore, for calculating bias, drain voltage is 12V
Id=Vcc/R1=4mA
We will choose quiescent point at Id/2 - 2mA.
By using these methods http://www.diyaudio.com/forums/pass-labs/228897-jfet-matching-sorting.html
I measured for my JFET BF244A:
Idss=5.43mA which is about right looking at the datasheet.
Vgs,off=2.19V - again, about right
Now I have to calculate what Vgs I need to have 2mA drain current.

Using upper equation. When I turn it around I can calculate Vgs and I get Vgs=0.86V
That means Vg=Vs+Vgs=6+0.86=6.86V
From that I calculate that in voltage divider I have values from the schematics. But in real life it's not like that. I get right gate voltage, but source voltage is always about 2V off from my calculations. I understand that I my measurement of Idss and Vgs,off may be wrong, but I don't have another way of measuring them.
I tried having 4V at source and then doing the calculations and in practical circuit I got around 6V. If anything, it's always about 2V and I can just do the calculations like that, but I would prefer to know what is happening here.
What am I doing wrong here?
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