If memory serves the lowest HF I see monsters getting is a 10, so usually for horrible but "reality based" visions I usually go with a 10 for h.f. or I may lower it if its something the group sees on a regular basis.
The wolf howling and other "atmosphere" moments in my game dont usually get a HF unless its specific to the groups danger (like the howling means werewolves are coming for them,etc.). There is a section in the BTS 1 book about using atmosphere to create the horror for the players, or at most say something like "The howling of the a wolf in the distance carries an unusual and near savage feel with it, sending a slight chill down each of your spines". You describe the horror and fearful moments and it causes the players to imagine it and create the horror themselves. You cant force the players into feeling fear or horror, you can on ly give them the opportunity to find it themselves.
The rules are set up so players get get bonuses to save vs HF as they go up in levels, this is the "desensitizing" part of the system. However, I have been one to lower or ignore HF for moments they've dealt with frequently and recently, and I've also added an optional house rule where a +1 bonus to HF is added at levels 3,6,8,12 and 15 to cover this angle.
Hope this helps out some.
P.S. I am rewriting the section from BTS 1 into the "Victims" sourcebook I'm working on and plan to submit as it seems appropiate and needed.
