I'm getting the same numbers you are now, though I was a bit more rigorous (read: anal) about the calculation. But actually if your intent is to damage the mob you're actually better off going with Careful Attack in the AC 19 case, because your chance to miss there is 80% (16/20) while the chance to miss with both double attacks is 81% (90% squared). But that's an extreme case. In the more realistic case of AC 14 it's very clear cut that Double Attack is best.
Math below, since I spent so long typing it out
[sblock]Assuming AC 19:
Careful Attack
5% chance of doing 8 damage
15% chance of doing 1d8 damage
80% chance of missing
One Double Attack
5% chance of doing 8 damage
5% chance of doing 1d8 damage
90% chance of missing
Both double attacks
Chance to miss with both attacks: 81%
Chance to hit with one attack only: 9%
Chance to crit with one attack only: 9%
Chance to hit with both attacks: .25%
Chance to crit with both attacks: .25%
Chance to hit with one and crit with one: .5%
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Now, assuming AC 14:
Careful Attack
5% chance of doing 8 damage
40% chance of doing 1d8 damage
55% chance of missing
One Double Attack
5% chance of doing 8 damage
30% chance of doing 1d8 damage
65% chance of missing
Both double attacks
Chance to miss with both attacks: 42.25%
Chance to hit with one attack only: 39%
Chance to crit with one attack only: 6.5%
Chance to hit with both attacks: 9%
Chance to crit with both attacks: .25%
Chance to hit with one and crit with one: 3%
[/sblock]