In Burning Wheel, the base obstacle of a test and disadvantages are treated separately.
(By "base obstacle" I mean the obstacle without conditional modifiers; this terminology might not appear in the book.)
The different treatment happens, for example, when there is double obstacle penalty - only the base obstacle is doubled, not the modifiers.
As a game master, how do I decide what is the base obstacle of a test, and what are modifiers? There are obvious cases, of course, but there are also unclear ones. The following are from the orienteering skill, and are just an example - I am not particularly interested in orienteering. Quote heavily edited for brevity.
Day and familiar land, Ob 1; Night familiar, Ob 2; Unfamiliar, Ob 3; Unfamiliar at night, Ob 4; Familiar during storm, Ob 5; Storm at night, Ob 6.
Here, it would be tempting to say that daytime obstacle is the basic one and nighttime gives +1 Ob. Or that a storm gives +4 Ob. Or unfamiliar land gives +2 Ob, etc.
Suppose I want to adjudicate killing a bunch of orcs with a simple weapon skill test. Should I say that that sounds a heroic thing to do, so obstacle is 6, or that killing one would be obstacle 4 and because there are so many, that is +2 Ob?
In general: How do I decide how the obstacle of a test is divided between the base obstacle and disadvantages?