Battery education -- How are you calculating mah?

I see people saying: I put back 400 mah. Maybe I am being real dumb, but don't you need to have a current draw to calculate th mah? So how can you know you are putting back a # of mah's? Wouldn't you be putting back a % of the voltage? Such as if you end points are 4.2v and 3.7v (.5v range), if you put back .25v then you would be at 50%.

Am I just not getting? I just got hyperion 550's and really want to keep them tip top! :)