Would a circuit like the attached be practical for this application? Using 1/2 of an LM 339 as a window comparator with a divided input the user could set a window of 28 Volts + or - 10%. The LED would only be lit when the applied voltage was between 25.2 volts and 30.8 volts. A known value of 28 VDC is applied and the wiper of RR6 is adjusted for 7.0 volts. Then the wiper of R3 is adjusted for 6.3 volts and the wiper of R1 adjusted for 7.7 volts.
The output could drive the LED or anything else like a small relay to prevent the 28 volts from being passed on unless it was in limits. The circuit is a rough and filtering on the 7812 wasn't included. I was just thinking that even if not a window comparator maybe a simple comparator could be used. It would offer a pretty clear On or Off (snap) for the LED.
It's late and 4 AM comes early so I am out of here.
Ron