I have a 4s with iso 9.3.5
I let the phone power off by itself when it got to 10%
Then i connected the charger, and turned the phone off so it can charge without interruption,
How will i know when the charge is 100% ? Because the screen displayed a battery with red in it, now its gone off.
Take the phone off the charger whenever you need or want to. You don't have to wait until it gets to 100%. You can also leave it on the charger after it has completed charging. That won't hurt it either.
Yes, your phone is charging even though the screen is off.
The process of calibration was to calibrate the battery sensor, not the battery itself. It is no longer necessary or recommended. The article anypats linked to gives more details.
If your battery is draining quickly, it's probably because it's very old. No amount of "calibration" will fix that. You'll need to get it replaced.