alan_m wrote:

Something you can buy at the local hardware store for less than £100 or
$100 is unlikely to have been individually calibrated or finely
adjusted. It is more likely to have had a go/no test after production.

There's good news and bad news in these numbers.

Bearing in mind that, for home use, a few percent error is ok, all you're
really looking for is a gross error.

How much is an acceptable gross error?
I don't know.

Maybe 10% for a head bolt or valve cover bolt?
Maybe even 20% for a lug nut?

Let's assume it's 5%.
All we need is a calibration standard that is plus or minus 5% then.

That's the trick.
Where do we get a calibration standard that will pay for itself?

A lot of people seem to use this $30 HF #68283 digital torque wrench
adapter:
https://www.harborfreight.com/12-in-...ter-68283.html
https://www.youtube.com/watch?v=EcOql9eK_Uk

Here's the owners manual:
http://manuals.harborfreight.com/man...8999/68283.pdf

If I didn't already have plenty of torque wrenches, I'd buy that, but I
don't need more torque wrenches as all tools have storage costs.
https://www.youtube.com/results?sear...nch+adapter%3A