That's around 20Nm, which is the normal specification for spark plugs with dry threads (20-25Nm).
Problem with torque values is how inconsistent it can be. To an engineer, the clamping force is the real interest. That is the force that the screw is puttin to clamp something together. To get the required clamping force from the screw, they calculate the torque out of the thread pitch and standard friction. Now, the pitch is not a variable, but the friction is. With no friction, the torque could theoretically be unlimited. There are tables which tell you the friction on various washer, on zinc plating or bluing or stainless or aluminium... Also the friction between the threads is very important, but als obetween the head/nut face and flange face, and these can be different. Engineers have to take all of this into account.
So, put on anti-seize on the spark plug threads, and the torque value becomes meaningless. Add on carbon deposits on the threads and again it ruin the torque values.
Of course, engineers perscribe a decent safety factor to counter this, but on a 60 year old alloy head sometimes it's just not enough. Tighten the plug in enough times and the threads get deformed too...
Also, not all spark plugs are created equal. Some have smoother threads, and some even have anti-seize coating on from the factory.
A well oiled M16 size blued screw on a washer compared to a dry screw on a washer, will produce a 40% higher clamping force. That's 40% more force on the threads, at the same torque value on the torque wrench. Spark plugs are around that size too, but with a finer pitch so the friction affects them even more!
So to counter all this, I also tighten to a torque angle, not a torque value. You feel when the plug crush washer is flattened, the resistance increases a lot. From then, I tighten around 1/8th of a turn more (45°), maybe even less, and that's it. Almost never had an issue with a leaking spark plug if it is new. If the spark plug leaks, you can always tighten it more - the gasket is metal so it does not get ruined if it leaks. And you'll hear and see it if it leaks...
For precise high tension screw tightening, instead of torque values they use a special ultrasound device. Depending on the frequency of vibrations in the screw, they can read the exact clamping force regardless of tightening torque. Those are accurate to over 95%. But the screw material has to be precisely determined. For ultra high precision tasks, there are also screws with a load cell inside (only larger diameters and these are extremely pricey purpose made fasteners, not sure if there's any mass production at all).
I wouldn't buy a used torque wrench unless I'd take it to calibration too. Get a 1/4" torque wrench, it's most useful for bikes. Maybe a 3/8" for cylinder studs for some bikes... A cheap one is generally precise enough if you never abuse it. At work we use Hazet digital torque wrenches but those are really pricey and I'd never buy it for home use...
