I notice that my altitude verbal limit alerts are not +-200 feet from target. The low alert comes on about 290 feet below target (bug) and the high alert comes on about 110 above. I think the 500 ft approach to target alert is right on. What can I do to adjust the limits. I like to use the low limit to help alert at minimums on IFR approaches, but I'm cautious. I'd like to know that whatever the spread is is either adjustable or not, and if not, that it will reliably stay where it is. Thanks...Jack L