Ignition Timing: How Automated?

When setting the ignition timing of 1990s Honda cars, how
critical is it for the fixed pointer to line up exactly with
the center BTDC timing mark (of the three BTDC marks) on the
crankshaft pulley?

Background:
The second step for setting the timing on my 91 Civic (as
well as other Hondas) is to jumper the "Ignition Timing
Adjusting Connector." I thought what this did was keep the
car's computer from trying to adjust the timing
automatically while one was monkeying with the housing etc.
Correct?

Then one rotates the distributor housing until the timing
light shows the pointer lining up with the center of the
three red marks. The timing light flashes according to when
#1 cylinder fires. The crankshaft will then be oriented so
that, ideally, I guess firing occurs--under these
conditions, namely idle, normal operating temp, no ECU
input, and so forth--at 18 degrees BTDC. Correct?

The Civic manual specifies 16 to 20 degrees BTDC at 675-750
RPM. It does look like the red marks farthest apart could be
about four timing degrees apart.

But what if, because it's so hard to see those pulley marks
(with middle-aged eyes; the grime on the pulley; and the
interference of the parts above the pulley), the pointer is
off and actually points closer to one of the two outside
marks? Will the computer correct for this? In other words,
when setting timing on modern cars (say 1990 and newer or
so), is one just getting the timing in the ball park, and
then the computer does the rest?

After all, the computer does have crank angle and RPM
measurements and I guess also firing inputs to it. It is
advancing/retarding the firing timing as appropriate for
different conditions, via input to the igniter as shown on
schematics, correct?

My mileage was down a bit (from 40++ to just under 39) for a
few tanks of gas through October and November. Granted the
temperature dropped unusually low for my parts. But I'd also
been messing with the valve lash and ignition timing a bit.
On the most recent tank, upon fillup the other day, the
mileage came back up to 41 mpg. Should I be all that
concerned about aligning that center white mark perfectly
with the pointer?

The best excerpt I could find on this topic appears under
the "tuneup" sections of the autozone manuals and my
Chilton's manual, and the UK site's manuals. See for
example:

 

Yes you disable the timing adjustments before setting the base timing.

The actual timing once things are hooked back up will be more advanced
than 18 degrees at idle. The lower the cylinder charge the slower the
flame front speed hence the greater the advance needed to keep the
starting and ending times for combustion roughly symetrical about TDC.

Never heard of self-correcting, closed loop timing, except for those
cars with knock sensors.

However I believe all adjustments made by the ecu assume a correct
basic mechanical setting.

 

if you want it to be "correct", the center mark needs to be exactly aligned.

if that's the case, it's not set right.

not on yours because tdc is determined by the distributor, not a
flywheel sensor like on later models.

not on any model without the flywheel tdc sensor.

only if it has an accurate [fixed] reference.

yes. too advanced means higher combustion temperature and greater nox
production, oil consumption, etc. too retarded means excess gas
consumption and lackluster performance.

i see no example!

bottom line, it all depends on the type of ignition system you have.
your vintage honda has all its ignition timing and injection timing set
by the position sensors in the distributor. if they're not correct,
everything is off, so try and get it as correct as possible. unless you
have one of the really high intensity kick-butt timing lights, set the
timing at night. that way, you can see the marks much better without
too much ambient reducing contrast.