Accurate voltage in transmitter

Alright guys,
so again my FC is SPF3 deluxe,reciever Turnigy IA6C and my transmitter is FS-i6.
In order to get my battery voltage in my transmitter I connected the B-Det from the reciever to the battery connectors.
I do get voltage in my transmitter: ExtV2.
But I have few questions
First, how can I get more accurate voltage reading in the transmitter? I have a 3cell battery and when I see 11.1V in ExtV2 I land the drone, put the battery in charger and see that there is around 0.15v~ difference in every cell, for example 3.85v,3.84v,3.85v.
Second, how I can get the alarm to work in my FS-i6 transmitter? which values I need to put? for 3cell and 4 cell becuase I didn’t manage to figure it out.
Thank you guys.

I’m not sure the voltage readings can be calibrated.
There always seem to be a voltage difference between different meters or different transmitters and lipo voltage readers.

Im not sure about the voltage alarms… I’m still googling…