[Mark_T]

May I point out that any five of you could torque that plug to what you think is exactly 22 ft-lbs, and no two of you would have acheived the same amount of tightness.

This was demonstrated quite effectively to me the last time I had my torque wrenches calibrated, when the tech showed me how your individual touch using the tool will affect results. Even though I could consistently get the same number on their meter, the tech consistently got a different result, and my buddy consistently got yet a third number. The variance between the three of us was about 4 ft-lbs at 50 ft-lbs, on a wrench with a range of 5 - 75 ft-lbs.

Also take into consideration the potential error present in the tool itself. When was it last calibrated? Is it a quality tool?

So, really, debating whether it should be 20, 21, or 22 lbs is pretty pointless. If, like JFP says, you go with the Porsche spec, your actual range of torque will likely be anywhere from 19 to 25 lbs, assuming a properly calibrated tool with average variance. And it will work just fine.