First off John, you would take a light skim off the rusty old bar to get yourself to clean metal. Then take a micrometer reading off the bar.
Depending whether you work in imperial or metric, you press a button on the display to put you into the correct units, then press another button to zero up the display.
Also depending on how you originally set the display up, either amount off the diameter, or like most people and lathes, amount off radius (1mm off radius takes 2mm off the diameter).
You can then do what John S has said, input that figure into the display and cut until your display shows what you want to end up at.
Or do it the way I do, a**e about face.
Subtract the diameter you want to end up at, say 15mm, from the mic measurement, again say 19.5, leaving 4.5mm.
By taking your cutter display into half that figure (2.25mm) in easy stages, once you reach 2.25mm on the display, your bar should be exactly 15mm diameter.
That does sound rather complicated, but in fact, once you get used to it, everything becomes second nature and you can cut down to your 15mm size continually, time after time. In fact if you zero your tool when you reach the correct size, no matter what size bar you put in there, if you cut down until the display shows zero, you will be at 15mm diameter exactly.
There are different ways of doing things with these display boxes, each type having various features you can use, even some that will help you to turn tapers.
Depending on the type of scale that is fitted to the display, governs how accurate and repeatable the cutting action becomes. Using cheap vernier scales you would expect your accuracy to be within 0.002" (0.05mm), whereas with the more expensive glass scales, that figure can become 0.0002" (0.005mm)
John