I don’t think you’ll be complaining on any of them. Was having gigabit nice? Of course, but 350 is plenty. Most services won’t even transfer that quickly to you and you can run a lot of video streams on that.
But the main way to tell would be if your router has a traffic meter on it since it has all devices going through it. Otherwise if you’re mainly on PC you can use the task manager to see how much is going on and out
It’s how CPUs do floating point calculations. It’s not just javascript. Long story short, a float is stored in the format of one bit for the +/-, some bits for a base value (mantissa), and some bits for the exponent. As a result, some numbers aren’t quite representable exactly.