Your method is good for determining whether actual, "but-for" causation is present. Proximate cause is subjective, and the best that can be said about it is that it encompasses some combination of foreseeability and "non-attenuatedness." You couldn't design a formula to determine proximate cause any more than you could design one to determine whether someone likes you or not.
Let me see if I understand this correctly. "But-for" causation is used for determining Negligence Per Se cases, while Proximate cause is used for regular Negligence?
"But-for" causation is always needed. (Well, maybe not in respondeat superior, but we'll leave that alone for now). Negligence per se allows you to get around proving duty and breach. You still have to show proximate cause. I don't think there's a way to get around proximate cause. Your four elements are duty, breach, causation and damages, and that causation element is legal cause, which includes "but-for" (cause in fact) and proximate cause.
Let's take an example. You are driving 40 mph in a 25 zone. Coming around a curve, you see me, but can't stop in time, and hit me, causing injuries. When I sue you, I don't have to make the case to the jury that you had a duty to me to drive at a certain speed - instead, I can rely on the statute, and your violation of the statute, per se, establishes your breach of duty. I only have to prove causation, which is easy, and damages, which should be as well, and rebut your defense of comparative negligence.
Another example. I am driving 40 mph in a 25 zone. You are my passenger. As we drive under a tree, a branch falls, goes through my windshield on the passenger side, and hits you in the head, causing injuries. When you sue me - well, you probably won't be able to rely on the statute for negligence per se, for other reasons, but let's say for the sake of they hypo that you can use the statute to establish negligence per se. You still have to show causation, which you won't be able to do. Although my speeding is clearly a "but-for" cause of your injuries (after all, if I had been driving 25, the branch would have fallen long before we reached the tree), it is not the legal cause, because, as the court might phrase it, it was unforeseeable
that my excessive speed would put us under that particular tree at that particular moment, causing that injury. You will likely not recover.
Note - I could be somewhat off on this. I booked Torts, but that was last year...
Back to your OP, the response was correct - your logical analysis is great for "but-for" causation, but won't help you in proximate cause.
"The life of the law is not logic, but experience."