Welding torch stepper motor driven auto Height controller.

I've been looking into the possibility of an arduino based automatic welding torch height controller. The basics are pretty simple. When welding if the torch moves away from the work piece then the voltage from the welding power source creeps up by a small amount & the opposite happens when the torch moves closer.
The target voltage is around 40V DC however I am do not think I can use a simple circuit such as a divider to monitor the voltage accurately (the arduino must react to changes of 0.1 of a volt). I think I will have to make some kind of op amp or similar circuit.
I know it can be done by buying an expensive controller which does this & outputs an up or down command, however I cannot justify or afford 400 pounds on a project which automates something which I can adjust manually.
I can manage the motor response, well, should I say I will manage that but I've always had problems measuring higher voltages accurately.
Just wondered If anyone knows a way for me to do this on a budget, I have a fair electronic knowledge.

Hi PaulEE,

What is the expected voltage range? 40 v is the target, but what is the min/max? 0-240v?