[Steve Tinker]

First off, check to see what drain you have when the car is stationary with everything switched off.
Do this by disconnecting the neg side of the battery and connecting your multimeter between the gound wire and the battery terminal. Put multimeter selector to mA and take reading - with everyhing off (including interior lights) you should see +/- 30mA. Anything much more and you have a parasitic drain which is slowly flattening your battery.

A fully charged battery should read 12.6v after charging and then applying a load (headlights) for about a minute as you have already done. 12.3v has the battery @ about 50% charged.

Alternator charge rate usually starts off +/- 14v, dropping to 13.6v after a few minutes running with a fully charged battery. Anything lower than 13.2v could be a faulty alternator / regulator.