How is AUTO db_file_multiblock_read_count calculated?

Starting 10gR2, standard recommendation is to leave db_file_multiblock_read_count unset so that Oracle can can calculate its value but I've got some Questions around it :

1. How ? : How does Oracle calculate it? I did a bit of Google digging and found a couple of hits - one from Jonathan Lewis saying it (actually _db_file_exec_read_count which it is deriving it from) is db_buffer_cache/sessions (init parameter) and then from Charles Schultz<http://www.blogger.com/profile/07973399674184183130> blog that its db_buffer_cache/processes (init parameter) - unfortunately its none of them exactly in my case. Here are the values :

db_cache_size : 7GBs
processes : 10000
sessions : 20000
db_file_multiblock_read_count : 42 (It was 43 before 10.2.0.5 upgrade a week before).

As you can see its not fitting into any of the two calculations perfectly. But I am not surprised because (actually I'd have surprised if it had worked that simply) its the parameter about how a system's IO is performing so there got to be something else in that calculation? Exactly my Question - What is that 'something else' and how is that calculated!?

2. Why I bother? : Everyone seems to say leave is as it is but my questions/concerns are :

i. How did it get changed last week? I need to know how it works so that I can anticipate when its gonna change and to what value from stability point of view. (I can only guess that the formula for its calculation changed in 10.2.0.5 but is this the right answer?)
ii. What is that 'something else' on which this parameter depends as I asked in my last question becs if we change that something else unknowingly we may end up in a surprise?
iii. It seems this parameter is set at DB/instance startup time - but thats not the best time to know how my system's IO is performing. I believe this parameter represents (or atleast intends to) the 'adjusted' or real value of my multi block gets - but that can only be known on a typical workload (when the buffer cache has got the usual stuff in it) and not at the instance startup time so is this value really reliable for the purpose its intending to fulfil?

Considering all this - Is Christian Angonini's approach of manually calculating this parameter the best then?

Or as usual am I missing something basic please?

Sorry I wanted to try it myself on my home DB but need the answer a bit urgently so been lazy hence resorting to your help.

Thanks.

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