How is the duration of a 60-day Covid extension calculated in Thailand?

It's 60 days not two weeks or Months it's 60 days

I have already done 2 60 days covid extensions, walk in 1 week before, and start every time from last stamp, never lost 1 day, in jomtien immigration, the first covid extension done after a medical visa

I’ve heard for the first one it starts the day you apply but I’ve just got mine and it added 60 days on form my last stamp, I applied 5 days before my last stamp expired

Depends. When you are coming directly from your original visa / entry stamp (i.e. if it is your first COVID-60 extension), they are counting from the day of application (so you may loose days if you go there early). If already on a COVID-60 day extension, the next extension will be added to the end date of the current one (so you will not loose days, if you go early). This is the general rule. Then again, TIT, nothing is applied coherently among different offices of the same organization, unfortunately.

60 days from the date the stamp is issued. The covid unable to travel extensions are different than normal extensions that are issued effective when your current entry stamp expires.