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Honestly, I typically just pull a few candidates out of my box of assorted springs, and try a few until I find one that feels right. After building a whole lot of extruders, I can kind of eyeball the spring and estimate whether it's stiff enough or not.
Probably more stiff than it needs to be: http://www.thingiverse.com/make:200447
Looks about right: http://www.thingiverse.com/make:176763
Looks a little weak: http://www.thingiverse.com/make:156324
Err on the side of longer/stiffer when buying unknown springs. If it's too stiff, you can cut off a wrap or two from the spring to loosen it up a bit.
I would guess that a ~25mm long spring rated ~8 lb/in should be in the right ballpark once compressed to ~15mm. The force applied at the idler is roughly double the spring force, so that would give somewhere around 6 lbs force on the filament. I would think that's acceptable...? If not, you can always print one of the adjustable bases and tighten it down.
It's on my to-do list, but I haven't yet created a calculator to relate idler pressure to drive gear stripping :-)