This is somethign I've been struggling to understand.
The answer is: depends on what book you're using. Different books give different systems, and its never been clarified, that I can see.
So far as I can tell, there are two ways that ACs are used.
One is a spell where the base level includes a sliding magnitude scale for distance, for example teleportation spells for humans. ReCo15 transports 5 paces, ReCo20 a little more .... ReCo35 to a point to which you have an Arcane Connection.
The other is for a generic effect that does not change dependant on distance. This includes spells like Pilium of Fire. Pilium of Fire uses the guideline for creating fire, which says intensity is equal to (level + 5). This means you can adjust the range to any form of spell. R: Voice is + 3 magnitudes; R: Sight is + 3 magnitudes R: Arcane is +4 magnitudes.
The same applies for Inexorable search.
Does this make any sense whatsoever? No. Not at all. Its wildly inconsistant. Which is why some teleportation spells (Wizard's Leap; Leap of Homecoming; Seven League Stride) use a sliding scale of increasing base-levels to adjust for distance and others (The Unfaithful Favour, ReAn20, HoH: TL) teleports a hankerchief to an Arcane destination, where it strangles someone to death.
All other spells seem to use Base level + Arcane. There is no rhyme or reason to it.
One way to think of it is this, though.
Every effect has a sliding scale of potency, regarding its primary purpose. Creating fire becomes more intense the hotter it is. Wind is harder to create the harder it blows. Emotions are harder to manipulate at their extremes. Since these are the primary purpose of these effects, secondary attributes carry a fixed difficulty.
Teleportation is harder as distance grows.
However, as there is no official explanation, this is strictly IMAO.