I tend to think of doable as qualitative, not quantitative – the game doesn’t expect me to track how many dice a player can concievably put into their pool, and any dice pool can potentially succeed with a Fate point. I’m not sure if those were terms you were thinking on, but it seemed worth bringing it up. I think it’s especially worth bringing up because it leads to the logical conclusion of not considering a player’s dice pool when calling for tests.
To go more toward your question, anytime a player says they have their character do something and I feel like calling for a test, I ask myself what the consequence of failure is. If I find that I’m thinking too hard, I say yes. Too hard can fluctuate based on if there’s a player’s BIT involved, or part of the setting/big picture I care about, or (in the other direction) if I feel like there’s better stuff coming up and failure would cause us to drag (better for everyone at the table, that is).
To give an example… The Questing Knights of the Grail have finally done it. The GM (that’s me!) says, “The old tome closes with a satisfying thud. Beren, you know the grail to be in the old Grathian temple – once opulent, now shrouded in mystery – at Black Garden, the city of the dead.”
“Fuck yeah!” calls Beren’s player, everyone else joins him. “Alright, boys, let’s mount the hell up, and ride!” Everyone emphatically agrees.
“Great,” I say. “Long forgotten Black Garden is only a week or so away, past the rolling hills of the Danov and --” I know what you’re thinking… As we all know, the Riding to Travel rules on page 533 of BWGR list a set of Riding obstacles for just such a trip:
Base Ob 1, +1 for a moderate journey, +2 for hilly terrain, +1 for riding Warhorses. That’s a total of Ob 5; I think you’ll agree that qualifies as hard in most cases?
And so now the GM (hi again) has to decide whether to call for a test or say yes. And the question to ask, as always, is, ‘What happens if they fail?" The GM thinks. And thinks some more. Then he thinks, "I’m thinking about this a lot. Does anyone have a Belief about this journey in particular? Or about their horse or being a good rider? No? Are the hills of Danov something I care about enough to require a test to cross? No? Do I want to get to my cool fucking undead city and my ruined temple to a dead god? Yes! I’m probably thinking about this too hard.’ And he says,
“-- and you guys pass all that, perhaps in a heady blur, so focused on the target of your quest. And now, before you stands doom citadel of ash and iron!”
If there was a Belief to engage (You love your horse? Well, if you fail, your horse is gonna get sick from you riding it so hard for so long), or if that part of my setting was something I felt demanded contending with (Oh, don’t worry, if you fail, you’ll just have to stop and camp out for a night… In troll country!), or if mundane challenges were tonally important to me (If you fail, you’ll arrive at my cool undead city ass-chafed and saddle sore! Buy some riding horses, ya cheap fucking scallywags!), then I might feel a test was called for – and probably I wouldn’t have to think too hard on the moment for a cool failure condition.
Some of this stuff is discussed on pages 72 and 73 of BWGR.