Setting by ear at idle only works for a start. When the advance comes in, you may need more or less timing. If it pings on accelleration, retard, if it bogs, advance.
Timing lights and book specs can't take into account the effects of age, wear and tear, and many small variables we introduce ourselves when we modify engines. I get them to run the way I want, then put the light on them and record the setting for future use.