That is where you are wrong. All it takes is for the arresting officer to decide that you are intoxicated. You could be totaly sober without any alcohol in your system and you could still be prosecuted with a PI.
You are right, the Texas penal code does not define PI in any measurable term. Only "-- may cause harm to oneself or others." For any LEO other than TABC, this usually means falling down drunk.
I bet if you chose to fight a PI half the time the cop wouldn't even show. Cops are corrupt idiots, basically.