JBuck, I've usually made do with a 6dB/octave high cut filter and simply dialed it in (i.e. adjusted the frequency roll-off point) until the HF attenuation sounds plausible - or like what I want - for the distance I'm setting up. In my book, 6dB/octave is a practical and reasonable approximation of the scientific data for frequencies above 1 kHz.
Distance without taking into account air absorption, on the other hand, is usually reckoned as 6dB reduction for every doubling of distance - assuming spherical spreading from the sound source.
Yes air absorption of audible sound is subtle within the distances most of us here are working with. For example, at 0% Relative Humidity and 20ΒΊC, air absorption is only about 5 dB/kilometre at 2Khz, and 30 dB/kilometre at 10kHz. (Unfortunately, hardcore scientific data sets on air absorption are locked behind paywalls and subject to strict copyright, so I'll desist from quoting any more data here.)
I see above that the "Air" plugin from Sound Particles has been mentioned (the "Air - Music Edition" version is free and good enough for our purposes). I have it and it's a potentially very useful plugin because you can use it either for air absorption only, or else combine air absorption-attenuation with normal distance-attenuation also. That said, I still use the simple old-fashioned single-pole high-cut filter solution, because like you, I sometimes want to play around with the HF roll off; but maybe for some purposes I'll switch over to using the Air plugin later on.