If i remember, when the explanation was given it was like this:
Base luck: 20%
Allan Bonus: 50%
So anyone using allan set would have 30% luck.
Base luck: 20%
Cartwright bonus: 50%
So anyone using cartwright saddle and horse you have 30% luck.
Base luck: 20%
Allan Bonus: 50%
Cartwright Bonus: 50%
So anyone using both sets have 40% luck.
With the Pat Desmond or Aztec weapons:
Base luck: 20%
Allan Bonus: 50%
Cartwright Bonus: 50%
Weapons luck set: 10%
So anyone using the three sets have 42% luck.
(luck is a value that nobody knows... 20% was used just has an example)