A phrase believed to originated in World War II, this is a military term which deals with the adjustment of instrumentation. Numerous elements of the instrument panel have or had adjustments which could be used to calibrate them, but the most likely candidate is the bomb sight which at the time was a fairly primitive device which lacked automatic calibration. A "zero adjustment" would mean that the bomb landed where the sight indicated. This is certainly the sense in which the term is used today; "zeroing in" on someone is most commonly done with a firearm. You can also zero in on a numerical figure; when an equation is balanced the difference between the two sides is zero. One prime example is the balance sheet.
Thanks to rootbeer277 for noting zeroing in on an answer.