Initial timing

The timing chain has nothing to do with TDC.
Base timing, in the grand scheme of things means just about nothing.
Your engine, at any given moment, has a timing need that is constantly changing, based on load and rpm.
To think that you could hit anything right by ear timing the idle, is, IMO, ridiculous.
Your engine has
a basic power-timing curve
a maximum power timing number that varies only slightly from one engine to the next.
an idle timing flexibility almost beyond understanding
a PT load timing variance of many many many degrees
a huge cruise timing target that is rarely met.
a deceleration timing requirement that is almost totally ignored
a need for altitude compensation, a need for fuel quality compensation, and a need to compensate for the engine's age.
To imagine that you can twist the D and meet any of these needs is shear lunacy.
But yeah, if yur just setting the idle timing, you can set that be ear. So what. It is the very least important setting of all. Most engines will be happy with 10 to 30 degrees of idle timing; that's a heckuva window.
By contrast, most engines will be happy with just 34 to 36 degrees of power timing, somewhere after 3500 rpm.
And most engines will want cruise timing of well into the 50s.
Which of these are you gonna tune by ear?

I tell you what, maybe I'm a dummy, but after 5 years/50,000 miles , I was still fudging with my timing, in an ongoing quest for this and that, using hard data to develop my timing curves. Oh, to think I couldda just timed it by ear......

I'll tell you a secret; without a computer, you will never optimize ALL your engines timing requirements. Without a computer,the very best you can hope for is to get the power-timing right, and let the rest be as close as it gets. Idle timing counts for just about nothing until it dicks with your transferslots.

But yeah, if yur just setting the idle timing, you can set that be ear.............