@MeneerJacco you might check this entry as well, which points to a small web page doing the calculation work reg. at+cpsms command.
Be aware, that entering the value via at+cpsms is only a request towards the network. The network decides if your values are committed. To check the network reply, use
and check the values via:
You can paste the reply in the above mentioned web page as well to get the values ‘decoded’.
My recommendation: set the active time (T3324) to a minimum and use the ‘RAI’ - release assistance indicator after sending data to shorten the RRC connected mode to a minimum. Then your device will enter PSM faster.