This is what I have connected to far:

With R1 being a 100K and R2 a 10K resistor.
This is where my questions start:
Should the connection from R2 towards GND go to the arduino's GND connector or somewhere else ?
Replacing the resisters with a 1M /100K pair should produce the same measurements. What are the benefits of using a larger resistor ? Ideally I would like to measure up to 50V safely.
The battery I am connecting this to produces up to 50V and up to 10A current. Is the suggested plan safe to use with this configuration ?
I would also like to measure the current flowing. I thought about measuring the voltage differential across a known resistor and work out the current from that. Again is this the best approach. What other options are there.