The problem with basing it on the opponents BAB, is the action, as written, isn't about trying maintain a defensive posture while casting a spell, but rather trying to cast a spell while maintaining a defensive posture. If the check fails, the spell is lost but the opponent still doesn't get an AoO, which means regardless of what the caster does, he maintains a defensive posture.
If you want to change it to something based on BAB, then the mechanic should be changed to where a failed check doesn't disrupt the spell but instead allows an opponent to make an AOO. If the AOO succeeds, then the caster will need to make a second Concentration check to not lose a spell after taking damage. If the AOO misses or he succeeds in the second check, the caster still gets the spell off.