15V

Since the voltage measured 13.1 V before starting, first thing I'd do is recheck the meter readings.
Only time a battery should show over 12.8 Volts is right after being charged it will show a 'surface charge'.
Wait a few minutes or flip the lights on for a second and then it should show more accurately.

Next thing to do is check the voltage drop while running.
Measure:
Alternator output (positive) to ground.
Battery positve to ground.
Voltage regulator 'ignition', that is the feed wire, to ground.

If there is a difference of more than 1/2 volt, then start tracing the voltage drop.
Measure between:
the alternator output terminal (positive) and the firewall connector,
Then the firewall connector cavity in and out (ignition)
then to the ballast resitor connection, etc.

15.06 Volts is a little high but not terrible, depending on the temperature. If its 80 or 90 F then yes its over 1/2 volts out of spec. But if its 30 or 40 F its only a couple of tenths about spec.