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For the median the formula is roughly 11.75 + 1.25*CR. While that's close to 15 + CR, the factor of 1.25 is important on the long run. It means that with full BAB alone you can't keep up with the increasing AC - you need to improve weapon enhancement bonuses, ability scores, class powers and probably buffs.
I like to think in "quarters" here, bonuses of roughly 0.25 per level. Each quarter gives you +1 to AB within 4 levels:
Full BAB: 4 quarters (+4 AB over 4 levels)
Medium BAB: 3 quarters (+3 AB over 4 levels)
Weapon enhancement: 1 quarter (+3 weapon at level 12, for example)
Ability scores: 1 quarter (+2 AB from +2 regular increases and +2 item increases at level 8, for example)
Class powers: 1 quarter (+1 from weapon training roughly every 4 levels, for example)
Buffs: varies a lot
If you can make it to 5 quarters (factor 1.25), you can at least keep up with the increasing AC - might be good enough for a secondary martial. For good progress you need 6 quarters, though.
There are also things which drag you down:
Power Attack / Combat Expertise / Deadly Aim: -1 quarter
Iteratives: -1 quarter in average, varies between -0 and -2 quarters (level 6 is harsh, for example: -0 AB and -5 AB means -2.5 AB in average, so about -0.417, nearly -2 quarters)
So you should aim to compensate them, get rid of them (sooner or later) or accept the reduced accuracy for other benefits (damage is the most obvious).