As has been discussed here before, the modifiers to Spot don't jibe very well with the modifiers that an archer can get.
For example, an archer with Far Shot has some ridiculous range with a longbow. But they can't see that far to Spot something?! Yet the ranged attack modifier is supposed to represent such things (such as the +1 for Point Blank Shot).
IMC, I have started using a rule that I saw mentioned here (sorry, I don't remember whose rule it was). There is a +1 penalty for every 10 feet of distance between the spotter and the "spottee".

But at the 100 foot mark (ie. after the penalty gets to +10), I drop the penalty to +1 per 100 feet. And at 1000 feet, the penalty changes again to +1 per 1000 feet.
It's backwards, I know. The Spot check should be easier up close and harder far away (the curve is reversed). But this makes Spot practical for close-up use, which is the normal case, but also makes it reasonable for a long-range archer to actually see his target.
As pointed earlier in this thread, this is all dependent on visual conditions: fog and smoke, trees, hills, and so forth.