Your post hasn't precisely said what incorrect result you are seeing, unless it might be in your Word document.

I am guessing it might be in your last calculation.

Suggestion: I find I make fewer errors if I simply copy the original inequalities into the If,
rather than trying to manually reverse the direction of the inequalities, make extra equations, etc.
In your code this would then mean I would have to swap the position of Pin and Pout,
but I would hopefully make fewer errors doing it that way.

Suggestion: I might look really carefully at the volume of the box you are using in your last calculation.
You are also calculating NumberInside/NumberOutside*VolumeOfBox. Is that really correct?
That is not the same calculation you were doing further up in your earlier examples.

Suggestion: If you find and correct some mistakes I urge you not to say "that was a stupid mistake"
and try to as quickly as possible erase every trace of that. Instead I suggest you try to think of why
that sort of mistake might have happened and what rule or practice or habit you might follow in the
future which would try to ensure that this class or mistake not happen again. With careful observation
and thought you might be able to avoid some or even most these kinds of errors in the future.