I use german torque specs...guedentight...!
But seriously, it's really just a feel for me, hand tight, then a little 'umph' more to snug em. You'd be better off leaving em too loose than having them too tight. I had one back out on me a few months ago. Scared the crap out of me cause I thought I'd really broken something...it was backfiring right out of the cylinder. Fortunately, it happened within 100 yards of my house, and I was SO friggin happy to find it was just a loose plug. 10 mins later I was back on the road. The risk of over tightening is damage to the aluminum heads.