I never use a torque wrench when tightening spark plugs. As things have been explained to me, you can FEEL when a plug is correctly torqued. I myself am only an average technician but have talked to many really good motorcycle mechanics.

The reasoning is something like this: An exact or absolute torque value can never be, well er, exact. There are many variables like thread quality, cleanliness, lubrication, etc. In addition the plug gasket may be new or not. An old gasket will already be flattened. A new gasket needs to be correctly compressed.

Anyway as I was told, tighten the spark plug until you can it "feel" it bottom or tighten. With a new plug one can actually feel the gasket being squished and then suddenly tighten. With an old gasket, it tightens immediately. From this point I turn just "a little more." How much depends whether is a short or long reach plug, an aluminum or steel head, etc. John Muir (VW Repair for the Complete Idiot) used the term to "grok" which I think translates into "to understand profoundly through intuition or empathy." Maybe someone else can describe this better.