I understand that for many repair shops "certificates" of calibration for measuring tools must be maintained...especially for government work...not sure if that is the case here or not? This is not probably "authorized" for many shops (in my shop it is:usa), but every now and then (usually after a t-wrench has been shock-loaded buy the shop floor!) I will test two or three t-wrenches against each other on the same fastener and check for any difference. For the most part, they always seem to "click" or "dial-up" to within a few percent of each other.
Regards, Bob