It is a lot like the gas mileage stated on the window sticker of a new car.
That's a good analogy. It may not be the best way technically, but it is most likely the way it is specified in the local ordinances, if allowable levels are specified objectively at all. Let's face it, if you have to answer to a noise complaint your primary objective will be to demonstrate that you are in compliance with community standards as stated in the local ordinances. No matter how technically superior an alternative way of making the sound level measurement one may devise, it is bound to be less convincing to responsible authorities hearing the matter than if done in exactly the same way a cop with a sound meter would do it.
BTW, ordinances regulating environmental noise rarely/almost never regulate source levels, but rather levels received by a receptor in a noise-sensitive location (school, library, hospital, residence, et al). For example, OP stated his club standard was 98 dB, and I'll guess it was measured at 9' per the old AMA recommendation (or 3 meters, as recommended by Steve Kaluf to be consistent with international competition rules). The received level can be estimated at receptor's location by assuming spherical spreading. By this estimation method, the spreading loss at 1/4 mile would be 20 log (1320'/9') = 43 dB. A fairly typical ordinance limit is 55 dBA during daytime hours in rural and suburban residential areas. OP's club limit on source level of 98 dBA - 43 dB spreading loss at 1/4 mile yields 55 dBA for the received level at that distance. Seems like a reasonable limit, if nobody that would potentially be annoyed lives closer than that distance from the flying field. Probably not good enough to leave it at that, though. If the need to defend against a noise complaint were to crop up, the club would be in a better position to argue they are in compliance with community standards if the levels projected by engineering calculations were backed up by actual survey data. Our club has conducted such surveys on two occasions, several years apart. We have had encounters with a chronic complainer - but have kept ourselves armed with the necessary data to defend, and have done so successfully.
Abel