The short answer,its a tool to make a comparative measurement.You are not trying to dimenionally check you fired brass to any dimension on a drawing.All you are trying to do is begiin with a number,any number,it can be .017.It does not matter.From that number you can determine how much you have set the shoulder back with your sizer die.Maybe you size it and it reads .019.That is a change of .002.That is what you need to know.
}The initial reading,.007,or .001 or .021,is simply to have a baseline to measure from.It is the change from pre-sized to post sized that the tool measures.
Its a good tool.Precision Shooting's Reloading Guide recomends them.Learn to use it and you will minimize case stretch.You will get maximum life for your brass.
If you are shooting a semi-auto,you will be able to control proper head clearance for safe,reliable lock up.
You are on a good track!!