More info is required here. What sort of machining operation are you doing? Is it .020 of a counterbore? Is it off the entire face of the plate or the side? What sort of material is it? What machine are you using? What cutting tool do you have? What measuring instruments do you have to verify that .020 has been removed? What is your tolerance range for this? Does the piece get heat treated after machining? Is thermal expansion a potential issue here?
More info would be needed to give you a proper reply but the standard operating procedure for most any operation is to establish zero, take a partial cut, verify by measurement, adjust per your reading and then finish to the target dimension.
What sort of job do you have that requires you to cut metal? Is this a regular part of your duties? What have you done to educate yourself? Bought any books? Which ones? Do you know how to calculate speeds and feeds? Do you know how to stay safe around machinery?
I ask all these questions because I'm trying to steer you in the right direction and give you a good answer that will establish good work habits. I must also state that the machine shop can be a very dangerous place and if you don't know what you're doing you can sustain serious injury. I don't want to see that happen if it doesn't have to happen.