How to calculate Test Code Coverage when we don't have access to source code

I am writing tests for famous API website - https://jsonplaceholder.typicode.com/

Flow of test case

Call users endpoint and filter particular user and retrieve particular ID https://jsonplaceholder.typicode.com/users

Then After retrieving the userid of particular user I retrieve all the posts written by User https://jsonplaceholder.typicode.com/posts?userId=1

Multiple post id are returned and based on that I retrieve various comments pertaining to post

https://jsonplaceholder.typicode.com/comments?postId=1

And Validate the comments response

How should I calculate the test coverage of my test cases?