I think he's asking so he can gauge the remainder of the pad vs. the
present wear rate from the time they were new.  The assumption being
that the wear will continue at the present rate, which should be a
pretty good one (assumption).

That would have been my 'guess' for a reasonable starting point.  However,
during my 'free' maintenance period the dealer told me my front brakes were
50% worn at 15K miles (Inspection I) so at Inspection II (35K) I just had
them 'automatically' replace the pads - they'd be fully worn, right?.

Took the old pads home and measured them, and front pads had 10mm left.
(This is how I learned my lesson about BMW dealer service - now use
independents)  I do about 80/20 highway/city driving, but don't think I drove
35K miles on just 1mm of pad, so that is the origin of my question.  A
thicker original pad would make the wear rate much faster - throwing off my
estimate of remaining pad life.

I now have about 92K miles on the second set of pads that have ~7mm left.
But driving habits have changed, somewhat, so I wanted to see if I could
calculate an estimate of remaining pad life with say, a 25% safety margin.

Frank

I'm still here.

But I don't measure new brake pads and then guess how long it will be
until I need to next look at them.  Brake pads are one of those things
that you inspect periodically and replace as needed.  I'd say you
should start out inspecting them every 6 or 12 months and plan on
replacing them 'soon' when there's about 1mm or so of meat left on
them.  Or, you could just rely on the brake pad wear warning light
thingy, if you like.  There's prolly no harm in doing that either.

I happen to have a set of new pads for a 318is.  They measure 11mm pad
thickness.  My Chiltons manual says they should be replaced when they are
3mm thick.  This means that at 7mm thickness that you are half way through
the life of yours.  It should be a couple of years before you need to
replace them.