And all I'm saying is that, even in a perfectly deterministic universe, such calculations are intractable in principle. (Moreover, not all perfectly deterministic systems are computable, but that's another can of worms.)
As to "choice"; let's say that I can go to work tomorrow or not go to work. I make a decison whether to go or not. What I'm saying is that the choice has ALREADY been made; it was made as soon as the universe began.
I don't see the philosophical difference, as long as I am my own simplest computer, because ultimately, nothing can predict me. Besides, if someone handed me a printout proving to me that all of my actions for the last three years were algorithmically predictable, it wouldn't change how I act or how I feel about myself.
Viewed a different way, if someone proved to me that many of my actions were instead decided by a truly random and unpredictable "Pop-a-Matic" in my brain, I wouldn't consider myself to be any more or less "in control" than if the "Pop-a-Matic" were exactly predictable. My decisions are, in large measure, a mystery to me in either case.
I may THINK that I have freewill,
That's as may be, but speaking for myself, I know I have freewill. That's because I define "freewill" as the process by which I make conscious decisions. Whether that's deterministic or random or something else, I can't tell you...but I suspect that it doesn't make any qualitative difference. You might be interested in my contributions to this thread.
If the guy is counting protons, whatever number he comes up with will be the number it is correct for the one path to continue.
You're still missing that point. Let me try one last time: whatever number he comes up with will be WRONG for the calculation to reflect the actual physics. Let "the path" go wherever it is destined; it will be as unpredictable as if it were random at some level. If a system's destiny is inscrutably hidden and unknowable in principle, can it really be called destiny?