Just to report a small problem on this web site:
I tried to register from this page : Build battery
On its form the captcha is not displayed, so impossible to register from there.
In an other hand there was no problem when i registered from the forum page.
i'm using chrome on windows 10.
By the way : thank you all for that great project, i have so much fun with mysensors !
I think i've finnaly understood :
1.1V is not a reference like an offset it is a reference as a max readable voltage
Analog Input will always return values between 0 and 1023
5V / 1023 bits = 0.0048 V per bit
1.1V /1023 bits = 0.001075 V per bit
so with 1.1V we are allmost 5 times more accurate. Moreover it seems that 1.1V will stay stable even if battery voltage becomes low. (so understand now why using it)
And that's what mfalkvidd explained : if we type in code : analogReference(INTERNAL); then voltage must never exceed 1.1V on A0 (it is different on an arduino Mega)
Now to choose R1 and R2 here what i've made :
A0maxV = VbattMax * (R2 / (R1+R2)
for me it means
1.1V = 8.4V * (R2 / (10^6 Ohms + R2)
so i use R2 around 150k Ohms
A0voltage = A0value * 0.001075
A0Voltage = VBatt * (R2 /(R1+R2))
A0Voltage = VBatt * rRatio
VBatt = A0Voltage / rRatio = (A0Value *0.001075 ) /rRatio
So here the magic formula:
VBatt = (A0Value *0.001075 ) /rRatio
Let me know if i'm wrong somewhere.
oops you are right i mixed both 
i fixed my error, thanks