according to a conversation i had with Byron Young of NYSDEC a few years ago, you measure a straight line from the tip of the nose to the tip of the tail with the two lobes of the tailfin squeezed together.
you should also keep in mind, however, that a straight line is not a tape following the curve of the fishes width as it measures from tip to tip. just as a boats length is measured stem to stern down her centerline and not along her gunnel.
bottom line - if it's that close you're better off releasing it.
Have to agree with OPINARI - if you have to measure it - it should go back. I don't keep anything under 30"'s (personal keeper size I've set for myself)or if I've already got fish in the freezer - it goes back regardless of size.
Have to agree with opinari and wader if you have to measure throw it back. A 28inch fish probably has not had a chance to breed, I also will not take a fish under 30' and really only take home 2 maybe three fish a year