I feel I should point out that the rule of a linear +1 DC on listen and spot checks per X feet of distance is completely broken anyway.
By a strict interpretation of those rules, a mountain could not be seen on a clear day, and thunder claps cannot be heard unless they are right next to you. You also have the problem of a sound loud enough to be heard for miles clearly, suddenly becoming almost impossible to hear 200' further away. Similar problems occur with vision. If the DC to 'spot' a mountain is -8000, then you can always see it at 80000' but suddenly it is nearly impossible to see at 80200' feet. That's kinda silly.
D20 works really well for combat in relatively small areas between small forces none of which is significantly above 20th level. Anything else, and you have to start fiddling with it.
I tend to cause the DC to increase in a semi-logrithmic fashion. For instances, if the DC increases by 1 per 10 feet, then I limit it to +10 at 100 feet, then increase the DC by 1 per 100 feet after that until 1000 feet, and then by 1 per 1000 feet after that until 10,000 feet and so forth.