As others have said, torque on bolt or nut is such an approximate way of estimating clamping force that plus or minus 5 is not something to be overly worried about, especially since just a bit of lubricant on threads can change the reading more than that.
You can use an eight point star socket to directly test one torque wrench against another.
For the cost of a calibration you can buy one of these using a 20% off coupon:
http://t.harborfreight.com/12-in-drive-digital-torque-adapter-68283.htmlthe one I bought seems within the 2% advertised accuracy, and you can return it if not.
By fixing a torque wrench horizontal in a vise, you can hang vertical weights off the centerpoint of the hand grip and do your own calibration. An accurate "fishermans scale" can also be used instead of weights to pull on the centerpoint of the grip.
