Steve,
Your should bring your oven up to temperture slowly over about 7 firings. That will let everything cure nicely. By the 7th fire, the oven will start to get pretty good, and you will see noticeable improve for the next few firings past that. The slow curing will minimize cracking and the mortar won't shrink or crack as much.
This is one of those things you can't push faster.
It definitely isn't a question of dome height or mass under the cooking floor -- don't worry about that.
James |