how do we calculate estimation points in agile code example

Example: how to point user stories

Story Points represent the effort required to put a PBI 
(Product Backlog Item) live. Each Story Point represents a 
normal distribution of time. We use Fibonacci Squence. 
For example,1 Story Point could represent a range of 4–12 hours, 
2 Story Points 10–20 hours, and so on. This time distribution is 
unknown during estimation. By using reference PBI’s relative to which 
to estimate, it is not necessary to know how much time it takes. 
You just want to have a rough indication of how much time the PBI will 
take to complete.