Usage patterns
〈人〉にとって好都合だto be convenient/favorable for someone
This plan is convenient for me.
好都合な〈事〉a favorable/convenient thing (condition, timing, etc.)
The fact that he couldn't come turned out to be a favorable circumstance.
〈事〉が好都合に働くfor something to work out in one's favor
The unexpected change in weather worked out in our favor.