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To calibrate, simply find some arrangement that lets you torque on the end of the wrench. Easy if you have a welder: simply weld an old socket to a bar. Another one is to jam nuts on a bolt; have the socket on the torque wrench and counter it with a box wrench. Just hold the other wrench and the union of the two to push the torque wrench against a scale. Note the value when it clicks, measure the distance on the torque wrench, and do the math.

In my somewhat brief experience, as long as the torque wrench zeroes properly, it's usually fine. (Adjuster loses all tension when you dial it to zero.) This is because they use Hooke's spring law as the principle of their operation. There are other modes of failure/inaccuracy but they are rarer.

P.S. I should buy one of the digital gauges. I have a HF click type wrench that drifted like 40 lb•ft off within just months.

I suspect since the "throw away" wrench read higher, the spring was 'stretched'. This happens from not backing off the tension setting after use to the lowest scale number and over time it stretches/relaxes. You don't want to go past the numbers and take all tension off.
Using calibrated instruments is an art. Without a calibration certificate, you're guessing that the unit is correct. Torque is only about 2-5% of full scale accurate because varies by measuring and use technique. Check your manufacturer's statement. DIY wrench turners tend to buy on price since we don't use them as much as a certified tech/engine builder. A real quality certified wrench is costly. You really need more than one to cover the normal SAE bolt schedule. Those torq "protectors" fit into spaces the wrench won't so have utility. I think with enough practice/years, most people can tell how much pressure to apply to most non-critical bolts but I won't go into the 'wet v dry' arguments. Don't use the impact wrench for tightening! They're too quick and too powerful for most auto fasteners and rechargeables get more powerful every year!
Torq measurement is necessary when a sealing/structural task is performed, that requires uniform clamping force. Bolts apply force by stretching, so that's why it has to be measured to get the right amount of stretch. Not needed for most tasks, low grades.
But since the device checks out with more than one other, it's probably 'accurate'. Need to test more than a few times to establish the 'precision'.
I have more trouble with the digitals not because of accuracy, but because it doesn't get enough use and when needed, the batteries are shot.(just the other day doing alloy wheel lug nuts) I usually take the batts out but they're button type and can still go bad in the case. Left in they corrode the connections. Same with my micrometer but it's LCD. The price of small batteries is thru the roof like most things now.
Below is what Snap-on says. Search the subject from bona-fide sources, not wiki this or that or Utube video for more info. Get a handy fastener size torq chart and post on your wall next to your thread tap/drill bit chart.

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This is what I'm worried about. I have a feeling that mine is on the high end.

I might just pick up some used beam types for cheap, plus a decent (sub-$200) digital wrench. I prefer to get things as close to correct as possible.

I prefer the beam wrenches because you can see how close you are to your torque spec.
For instance, when re-tightening head bolts, are they all consistently 10 lbs under or are some 5 and some 15?
They do have the disadvantage that they are not ratcheting wrenches so getting enough clearance to turn a nut is sometimes a problem.