A skill-test succeeds if the roll is equal to or lower than the target-number. But your degree of success will be higher the higher you roll. So you must roll equal to or lower than the target-number in order to succeed, but as high as possible in order to increase your degree of success.
The value of rolling 00 is zero and not 100. At any time both dice come up with the same number; 00, 11, 22, 33, 44, etc; you'll be meet by a twist. Regarding a roll of 00 and 99 the twist is always respectively a critical failure and a critical success also no matter the modifiers.
Your "degrees of success" is calculated by reading "the tens digit of your successful dice-roll". And your "degree of failure" is calculated by subtracting "the tens digit of your failed dice-roll" from "the tens digit of your target-number"; the result is your "degree of failure".
If the result of calculating your degree of success or failure is zero you end up with a standard success or failure. In the rare circumstance that your target-number is 110 or more, you may add one "degree of success" per 10% above 100, on a successful skill-test.
In certain situations your Storyteller will tell you that you have advantage or disadvantage. When this happens you add +20 (advantage) or subtract -20 (disadvantage) from your target-number. A commen way to gain advantage is when your competence is deemed ample i relation to a certain challenge. Or if circumstances are obviously in your favour or disfavour.
Using a fate point will add +20 (not +10) to a test (DH, p. 293).
WHEN TO RULE & WHEN TO ROLL:
The system is built to persuade players into storytelling rather than dicerolling. That is unless you find yourself in combat. Most other situations are resolved without rolling the dice. And when you finally find yourself rolling it's rarely for difficulties above "challenging".
THE DEFAULT RULE:
Whenever your storyteller deems it appropriate. You succeed with a degree of success equal to 1 per 20% of your target-number instead of rolling the dice.
THREE IN A ROW:
In certain deadly situation you can actually die from missing three tests in a row. Here are some examples: climbing a tall and step cliff, holding you balance on top of a moving valkyrie, social interaction at the court of the sector lords, etc..
[ Release of DH2's "Enemy Without": 29. october 2015 ]