Ah, Grasshopper, no one knows. You must find the answer within you.
Or
The best you find anywhere is a starting point. Materials are different and different pieces of the same material can be different. Machines are different. In some processes weather can make a difference. There can be a difference between Monday morning procedures and Tuesday afternoon procedures.
It is horribly complicated and the more precise you get the more complicated it becomes.
There is a real reason you need, highly skilled, expensive people to run “automatic” machines.
you might look the handbook of machining.
author: Boothroy.
this´s a excellent book for this item. you can learn many things about machining and tools machines.