Yea. I've seen in many questions that they actually state "use 30 feet per millibar" or "use 27 feet per millibar". To have two answers that are correct for each of these is a bit evil imo. I guess 27 is the more correct answer, but it depends on where you are in the atmosphere.
96 x (absolute temperature (273K + actual temperature) / altitude pressure (millibars))
So at sea level in ISA conditions that's 27.3 feet.
So if you have the choice, and it's correct to use 27 feet, I guess that would be the "more correct" answer (at low levels). At 18,000 feet it's closer to 50 feet per millibar
Someone correct me if I'm wrong