k-multiplier explanation

i need this one fully explained as you can well understand after seeing the file that i only need to understand the concepts .
i am attaching excel workbook as well
I have used k = 2. But randomly. Author used k = 3. I don't understand why.

Why +/- ks?

How does this regression with these limits make a good prediction? Thanks.
you can code example so that i can understand better
how much ?

see the question carefully, its not at all assignment request , its just for my understanding

3 to 4 pages will be good