What about taking the muzzle volume and then working out a distance required to reach 10 or 15 dB volume? So if a volume is 100db at the muzzle, and you go 100m, it will be 4/3.pi.50^3=8,333,333 1/2 that is 4,166,666m3. (half because we don't count ground)
If you assume muzzle volume to be in a sphere of 1/2 1m3, that'd be 0.5m3.
Divide the far distance by the close one gives you 8,333,333, so you'd assume the sound is that many times quieter?
Just count backwards multiplying by 10 to go down in dB, so to go down 10,000,000 you go 7 steps, (10^7), and that's 80. 90dB would be 1,000,000.
So, umm, 8million times quieter than 150dB would be somewhere between 80 and 90dB, closer to 80 than 90. Maybe 82 or 83.
And you could work it like that to figure out where it would be 15dB.
Just a rather large WAG though. If anyone can test this it would be awesome.
Of course that would be on the plains, and assume sound spreads out evenly, but those are trifles.