Mark,
Welcome aboard the wonderful world of artisan baking. First, Pain a
l'Ancienne is more of a methodology than a bread type as such. It
happens that you are making a Cibatta with it but you can also make
baguettes, bread sticks, focaccia, and pizza as well. (Pizza is
defined as bread with something on it.) So, let's trouble shoot this
process for a bit.
Cibatta is usually a good size loaf and having a bit more dense crumb
is to be expected on the bottom part. Also the uneven holes may be
due, in part, to the structure of the bread setting before the oven
spring, or expansion after placement in the oven, is completed. I
would suggest that you preheat to 450F. and then drop to 400F after
you place the loaves on the stone. Also check your oven with a
thermometer to see if it is at the set temperature. Also, put the
stone on the next to the bottom level of the oven with the steam pan
below that. I have found that spritzing is not needed if the stone is
substantial and the oven retains steam for a reasonable period.
As for the bottom it should brown if given enough baking time. It is
actually harder to over bake bread than to under bake it so lengthen
your baking time. You can use a metal stem thermometer to check the
loaf and if it is at about 200F. the bread is done, if below that it
goes back in the oven.
You did not state but you should be retarding the dough at least
overnight and you can let it go for up to about 3 days if
necessary. This is the time that the full flavor of the flour is developed.
Hope this helps
Pop