Ok no problem with resistors I have 1% tolerance ones.
I used 0.1%, the best I could get. Not cheap but the measured voltage is within 0.5V of what my multimeter says. 0.5V in 50V is 1%, which is good enough for my purposes.
As for the voltage reference diodes will a TL431 suffice?
The problem is that it is adjustable, which means you have to adjust it, which means you have to have something accurate to calibrate it against. It depends how bothered you are about accuracy...
I can't tell you what voltage reference diodes to use. I don't have a note of what I used, other than they were precision 2.048V reference diodes. Maybe something like 2.048 reference. I don't know what availability you would have in your country. The precision and stability of the reference and the precision and stability of R6 and R7 is very important.
Sorry for the GND, I meant if the negative connection of the supply of the pic is "ok" to be in +0V (wich in your BTS power supplys also is connected to GND) I'm a little confused with the polarities that´s why I'm asking again and again. But if it´s ok like specified Ill try that.
There's clearly something you don't understand but I cannot work out what it is. There is no '+0V', there is just 0V, often called ground. There is only one 0V.
You also need to be aware that I was using a PIC with an internal 4.096V reference. I've never used the A2D on a Uno, I think it's reference is 1.1V. You have to adjust your divider circuit to take that into account.