I spruced up the spreadsheet I've mentioned in other posts and am making it freely available for others to use. Here is the link to download it:
(UPDATE: 2/25/2015 - New Version 2.0 at the Link Below!)
JFET Spreadsheet v2.0
Basically you provide your JFET's Idss and Vp (which I call Vgs(off) in the spreadsheet) and the desired source voltage Vdd (e.g. 9V). You then have several choices:
(1) Midpoint bias.
This assumes the drain current is 1/*Idss and Vd(bias) = 1/2*Vdd (i.e. 4.5 V drain voltage). A good reference point.
(2) Known source resistor and drain voltage.
Supply those values and it calculates the drain resistance and other parameters. This is what you can use if you have an amp sim (like one of the Runoffgroove circuits) with a known source resistance and target drain voltage (e.g. 4.5 V). You could use this to determine a single drain resistor which would bias your JFET (approximately), or you can just make sure you have the right size trimmer. Note that your stage needs to have the common source configuration for this to be valid (mu amp and other complex JFET amp circuits do not apply).
(3) Known source and drain resistors.
In this case, the calculator figures out the bias voltages (Vd, Vs). If these look out of whack (i.e. 1 V or 8V), then try a different JFET (Idss, Vgs(off)). This is what I used to determine what JFETs would work in the Plexi-Drive. Note that the Plexi-Drive uses the same source and drain resistors for each stage, so having a matched set of J201s makes sense.
(4) Fetzer Valve Bias.
I put this in for comparison purposes using formulas at the Runoffgroove site. It is interesting to note how different this is from other types of biasing. Is it better? Only your ears can say. It does seem to have less gain than other bias points.
Please note that the gain calculation (Av) is a crude approximation, and is mainly there for comparison purposes. If someone wants to enhance this with more complicated formulae, feel free to do so. The main thing is that you're getting some amplification (and NOT one or less than one!).
Have fun, and let me know if you discover any mistakes or problems. There is NO warranty expressed or implied, because it's free!