Yes, you're over-thinking it again... way too much. The small percentage that it may be off at the lower torque levels you're using, is really not going to matter. Even if it were off by 10%, most systems are engineered to regularly handle 115% of rated and are usually spec'd to 150% of rated. You could actually torque your lug nuts to 100lb-ft and probably never see any adverse effects. Think of how many people just go as tight as they can in lugs, if it was that critical you'd see cars down all over the place. You're also not providing work that could be called back for warranty repairs... the biggest reason shop torque wrenches are calibrated often.

For regular intermittent use, it's not cost effective for you to have it done. Mechanic shops don't have the equipment, but there are tool vendors (Proto, Snap On, etc) that either provide the service or have a local provider. They have specialized equipment to verify torque and make adjustments.

When I worked in a shop (granted, I'm large industrial, mostly engine building), the tool guy came around once a month trying to sell tools. He would collect stuff to be checked/repaired/calibrated and would usually return them the next week. Offshore (oil & gas platform in the Gulf of Mexico) we have multiple sets and send them in on rotation (we have them in in-lb all the way to up over 1000lb-ft).