From what I see, they have similar standards, different only for the top values.
- Under -70 means decent
- Under -80 means bad
- Under -90 means very bad
Value -30 seems to be the maximum, so you have to translate the range -70 to -30 into two more bars. You could choose -50, the middle, for example.
From a UX point of view, since the user doesn't know exactly what those values actually mean and since the wifi signals will not corelate directly to app performance or download speed, then your wifi meter becomes just a simple convention in your app.
As a rule of thumb, low quality values are more similar to each other than high quality values. In other words, if something is bad, it doesn't matter if it's "less bad" or "more bad", because it's still bad, so it's something to be avoided anyway.
But when it comes to something being good, that's when the user starts to care and distinguish between good and very good. If your meter says that a service is "very good", then the user will have high expectations and will be more frustrated if those expectations are not met accordingly.
Therefore, when your indicator shows the maximum wifi bars, then you should be certain that the signal is actually very good for most use cases. For a signal with values from -70 to -30, I would reserve the maximum bars for something very close to -30 (like -50, or even -40), even though it might rarely be achieved. In most situations, your indicator would show only 4 bars and rarely 5 bars, even for very good signals.
It's better to under promise and over deliver, than to betray your user's expectations.
