Probability of Default from Altman Z-score

I know how to calculate the PD using the Merton Model, Logit or Monte Carlo Simulation, but how do you go about calculating it from Altman's Z-score? I've read somewhere that it can be done by mapping the score to a rating and then inferring the PD from this rating but how is that done?

Also what are some other methods used to calculate PDs ?

Thanks a lot.