The latest official Community Q&A explained how your chance to get a critical success when crafting is calculated. I think it’s important enough to repeat here:
DarthTexas: Can you give some insight to the numbers behind crafting? What is the baseline crit chance for crafting? How does companion affection affect crit chance? Exactly how much of a boost is +1 or +5 critical chance in crafting?
Patrick Malott (Systems Designer): The baseline crit chance for crafting is dependent on the difficulty of the crafting action relative to your current crafting skill level. The difficulty is color coded in the crafting GUI.
Orange Difficulty: 10%
Yellow, Green, and Gray Difficulty: 15%
Companion Affection scales Crew Skill chance up to +5% at maximum affection.
A Companion Trait critical chance bonus of +1 or +5 is a percentage. Example: The Imperial Agent companion character Kaliyo grants a +2 bonus to Underworld Trading Critical. This is +2% bonus.
Using Kaliyo at maximum affection as an example, if she ran a green difficulty Underworld Trading mission for you, she would have a 22% chance to score a mission critical success.
Commentary (SWTOR-Crafter): I know some people will be disappointed at how small the bonus is for companion specialization, but I think it’s a good choice. If the companion bonuses were higher then players would be punished by not choosing the optimal class-skill combination. This way obssessive optimizers can get a small benefit over those who can’t be bothered and more casual players can choose whatever they want without having to worry too much about doing it wrong.
by Mike Searle [Prima Games]
by Drew Karpyshyn [LucasBooks]
by Pc Games [Electronic Arts]