Presumably like you, I do not usually need formal calibration, merely confirmation that it is giving adequately accurate answers. As discussed here in the past, many year ago I put together 'a box' to do this. Like you, IR and low resistance was done with resistors, and voltage just by comparison with other meters. Although not perfect, it checks the loop testing functions by inserting small value resistors in the loop path and seeing if the measured loop impedance changes by the appropriate amount. Checking the RCD functions is the most difficult, and I tend to utilise a 'reference RCD' (or, when the opportunity arises, compare it with someone else's machine).Out of interest @JohnW2 how did you calibrate your?
I did my loop tester with a couple of reels of t&e and a data sheet, the ir tester with some resistors I had lying around and the loop impedence tester by comparing with a visiting dno employee's unit. Luckily he had to try most of the sockets as they were mostly bad so I got a good range
Kind Regards, John