There are so many variables involved that any given number is just a basic guideline and nothing more. I've never known or read of a death actually being caused solely by caffeine and I doubt it's really likely/possible. I would imagine the body's defense mode would kick in somehow to prevent too much from being consumed at a given time. The '400 mg' safe zone is barely a starting point to my day. I can easily consume upwards of 1 gram in a 3-4 hr. timeframe and feel nothing out of the ordinary. No change in blood pressure, pulse, agitation, nothing... Not just guessing with my number either as my typical morning averages 5-6 doubles (espresso) in a few hours and often followed up with 1-2 energy drinks that are 160-240 mg each. I do feel what energy I have is simply sustained, but it takes quite a bit to actually feel energized on an unusual level. Honestly believe at this point in life I would have to consume upwards of 2 grams daily to feel any sort of a real jolt.
I do take health seriously as for the last 3 yrs I've battled a very serious near fatal illness that led to life altering/permanent changes. Also caused 2 small strokes at which time I cut my caffeine intake in half practically overnight just to be sure things would smooth out. The strokes had absolutely nothing to do with caffeine, blood pressure, etc., but I was doing my part to keep things in check. Once scans/labs were done to confirm and I started feeling somewhat normal again I did slowly bump up my caffeine intake to get back to my daily fix level. Told my neurologist that I was not ever intending to give up caffeine as it's the one thing that never lets me down and they totally agreed. Saying my system can handle what I do with no apparent problems and they had no concern on any changes to my daily regimen. When neurology can't suggest any room for improvement it sorta speaks volumes.
Point being we are all different with what we can tolerate and the only sure way to know is to try things to see what the end result is. Don't just go by what you read or hear, etc.