I'm not sure,but i think the bad readings had to do with the multimeter and also the fact that when i measured i had a ftdi cable connected. ( i mean, connected to rx/tx,dtr...)
I tried with another multimeter, measured without the ftdi ( fortunately, i send by radio also the battery voltage,so i can know it withouth using a ftdi to read serial port) and fixed the magic constant, and now the reading seems exact in two decimal points.
Thanks @Wawa and @jremington . Your confidence in the internal reference method made me to retry again.
That is reassuring! Thanks.