It looks as though you are trying to measure current not voltage. Is that right?
On your circuit, you are making things a lot more difficult for yourself by having such as small resistor for R4. You are also making extra work by connecting one side to ground, rather than your reference voltage.
In the maths on the microcontroller, it is likely that there will be a non-zero ADC reading when there is no current. If we call that X0, and the reading is X1, then the rms voltage for N samples is:-
Vrms = sqrt(Sum((X1 - X0)^2) / N)
So if the zero reading is 1000 counts, and you have readings of 1001, 1002, 1003 and 999,
then the offset readings are 1, 2, 3 and -1
those squared are 1, 4, 9 and 1
the sum is 15
the average is 3.75
the RMS value is 1.93
That has to be divided by however many counts above 1000 you get for 1 V input.