I've been calibrating torque wrenches (and a bunch of other stuff) for over 11 years. If you want quality, go snap-on. Or CDI. They seem to be the most reliable and repeatable. They're more expensive but you get what you pay for.

If you want your stuff calibrated you could do the weight @ 1 ft. assuming ft-lbs. Of course, you have to assume the weight is calibrated (which it probably isn't) but you'll get a "close enough". Make sure you check cardinal points at the 20%, 60% and 100% Full scale value. Also be sure the weight actually hangs freely and the handle is horizontal. Yes you also have to factor in the weight of whatever you use to suspend the weight.

Just by observation over the years technique plays a major factor in using a torque wrench.

To get best results...

- ensure there is no cosine error. This means make sure the wrench is mated completely flat to the socket and when you turn the handle it is completely perpendicular to the object being torqued. A change in an angle other than 90 degrees will change the torque value.
- Always store on the lowest settings. As one poster mentioned earlier it will change the linearity of the internal spring. Usually this is permanent.
- Before use, adjust to the highest setting and exercise six times. This will lube up the internals and give you more repeatable readings.
- always use a consistent, slow and fluid motion. Stop the instant you hear the click if it is an adjustable click type wrench. Anymore will surprisingly put much more torque than one may think.
- KEEP IT CLEAN!!!

These are probably the best torque wrenches I'd ever recommend. (no endorsement intended)

https://store.snapon.com/ADJUSTABLE-...E-C629532.aspx

 

I purchased split-beam style torque wrenches by Precision. No need to zero this type between uses.