How hard should it be to take out a guard in one shot? What's the genre? Low fantasy with not-so-competent heroes: almost impossible. Gritty fantasy with expert assassins going after powers-that-be: moderate. High fantasy high powered characters sneaking into a kobold stronghold: automatic success.
If you don't have an instant answer to that question, use the pass/fail cycle: if you need to justify a hard resistance, maybe there is a nasty wind or you just figure that unless they nail him right in the eye he has time to scream out and alert others; if you need to justify a low resistance the guard is drunk.
All that said, I think there is a secondary issue here: since the book doesn't tell you how long a range a bow has, or if a dwarf can outrun an angry bull, etc, you need to make sure you are all on the same page. This is important esp. re. character abilities: when someone picks "Longbowman" as an ability, you need to have some sort of agreement on longbow range in order to be able to consider credibility in the first place -- it doesn't need to be exact, but you need to talk about it briefly.
GM: So, you have Longbow as an ability. How far can you shoot, do you figure?
Player: Hundreds of yards!
GM: Ok!
Cheers,
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