HOWEVER, anesthesia is a state of unconsciousness that is adjusted to the patient and the surgical procedure. There are no formulas or paradigms that one can assume and "dial in".
An anesthesiologist provides to a patient what they "need". The need is determined by what happens in the operating room, what the surgeon requires and what the patient can tolerate. There is no auto-pilot.
As to your fear, I have no magic bullet to share with you to take it away. HOWEVER, keep in mind that the day you have surgery, that day you are likely to drive or be driven to the surgery center or hospital where you will have surgery, and being in a vehicle is way more dangerous than having anesthesia, and yet you probably get in a vehicle regularly and never give it a second thought.
Life is filled with hazards, but back surgery is done all the time, though there are many types. For perspective, I have provided probably close to 20,000 anesthetics in my career and no one has ever died in the operating room OR under my care. I suspect your anesthesiologist should offer a similar experience. In life, bad things happen and we control very little, but anesthesia care should be overall safe for you.