I swear, every time I think I get it, I end up being completely wrong. Basically, I'm still confused as to how , or , or this:
How did become 2.575? I understand that
...but not how that transforms into 2.575.
There are (at least) two types of normal table one gives the area under the curve from -infty to z, and the other the area from 0 to z (z>0). You want the former, but you can get the numbers by using the table you have got but subtracting 0.5 of off the p value (this is because the area from -infty to 0 is exactly 1/2).
So in this case you want to use p=0.995-0.5=0.495
RonL