BTW in one of the comments Bill Highleyman (co-author of the Breaking the Availability Barrier) pointed on the mistake in my formula which I corrected by replacing "n+n" with "mn". He also provided the excellent resource about availability calculation where he writes articles at the The Geek Corner for Availability Digest. One of the articles there extends the subject of this ( and couple previous) post and called: "Calculating Availability – Redundant Systems "
As I suspected my formula ("Trubin law") is just a particular case of more generic rule Bill Highleyman formulates in that article. That says:
"... Adding a spare node adds the number of nines associated with that node to the system
availability but reduced by the increase in failure modes.
That is, adding an additional spare node adds the number of 9s of that node to the system
availability – almost. This improvement in availability is reduced a bit by the increase in the
number of failure modes in the system. More nodes mean more failure modes..."