It is printed on the back of the SRM. Old training files should show what old slope was-

As others have noted, the 532 is a value in Hertz (Hz). This goes back to the days when SRM sent the data from the hub to the head unit using FM (frequency modulation). It was (is) not possible to send FM data at zero Hz, so all SRM units were designed to send something around 500 Hz when they were unloaded. Then, the driving torque in N-m was modulated onto the signal using a certain gain constant (in your case, 30.5 Hz/N-m).

In today's ANT+ bike profile compatible transmitters the signalling and encoding is completely different, but SRM still uses the same terminology and units to encode their data. A zero offset of 532 Hz is completely within spec, and you should only be worried if you see drastically different values from ride to ride. It is good practice (best practice!) to do a zero offset calibration about ten to 15 minutes (or more) after you get your bike outside, to ensure that the head unit has a valid zero offset number to use for the power computation.

Most likely, your head unit is configured to allow "auto zero" which means that anytime you coast, the bike computer reads the zero offset value that the SRM is then transmitting, and stores it for future use. Unfortunately, this method is not completely foolproof and occasionally you'll get a bad zero, and consequently weird power numbers that may throw off your training or race results. This is the reason that some riders and coaches will suggest you disable auto zero - but then it is imperative that you take care of that on every ride. Your data is only as good as the effort you put into getting it.

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Less is more.

Ah cool, thanks for the reply. Underlying message is I'll get a bit better at doing a zero offset at the start of my rides. To my mind, the data I've gathered over the past four years has been pretty consistent - I.e I've not suddenly noticed a 100 watt pb one day etc.

You should do a zero offset before every ride. No ifs, ands, or buts.

You should be able to set the edge to prompt to calibrate when the srm is detected at startup, it's under sensor details