Be interesting to see if the calibration is spot on as its a pain removing all the dust caps dragging the airline round to find out the tyres are all fine.
There are two things at play here, the accuracy of any pressure measuring device and the second reason for such a device being in use, a trend on each tyre.
The second reason is perhaps the easier to deal with, it's an indicator of a trend. We're all aware that the pressure in a tyre will probably drop over time, possibly months, and here the actual pressure indicated is of little value, but the rate at which it is observed to drop should indicate a fault, either a puncture, a badly seated bead, corrosion of the rim at the bead (a real problem with alloy wheels) or even a faulty valve.
The first reason, the accuracy, that will also depend upon the accuracy of the air line at the garage or whatever you used to inflate and check the pressure in the first place, bearing in mind that there is a "calibration tolerance" which must be allowed in the first place.
Say, for instance if you inflate the tyres to 30 psi at the garage and the TPMS then reads 30 then all is well with the world. If it should read 29 or 31 psi, so long as you're aware of the error, then all is still well with the world.
If you're going to have to remove all the caps and drag the airline around whenever you do decide to re-inflate the tyres.
And anyway, as I said in my first post, I bought it for no particular reason other than I would like one.