How Does Windows 8 Show 20% Lost Battery Capacity

As a student I had quite a lot of trouble with my previous laptop's battery. It had an autonomy of 4h in the beginning, but one day, while it still had 50%, it suddenly dropped to zero for some reason, and after that incident the battery has never exceeded 50% charge again. So I had to buy another one, but this one also dropped to 2h autonomy after some months, although it still showed 100% charge.

I now have a new laptop, an Asus T300LA, that I chose for its 8h autonomy. As I really wanted to take care of the battery, I searched on the internet how to deal with a li-ion, and learned that I had to drop the charge from 100% to 5% at first use, but should never let it go below 5% as it would permanently lose 20% of its capacity at once.But during the process I was too scared that the charge level shown by Windows would be false and while it would indicate 5% it would actually be less, so I plugged the laptop at 8%; and I was indeed right, as the battery level suddenly switched to 5% when I did so !

Basically I am fearing the infamous 20% battery damage occured without my knowing since the battery level indicated by Windows can't be trusted. The problem is, I don't know how to check it; with my previous laptop (Windows 7), on one battery the damage wasn't shown and it would still indicate 100% charge when filled, whereas on the other one the damage was clearly noticeable as the battery level would never indicate more than 50% even when filled up.

How can I verify if my battery is pristine or not ? If it indeed lost 20% of its capacity, should Windows 8 indicate a completely filled charge as 100% or 80% ?

Also, with maximum screen brightness Windows tells me this: "your current brightness setting might reduce battery life". What does it mean by "battery life" ? The current charge length or the whole time I can use the battery before it's good for junk ?