copyright grey area

[ledzep]

After careful examination of the copyright law it is clear to me that facts are not copyrightable, only the expresions, so my doubt is how much of the original expresions can I use without infringing the copirights of the other author, 50%,
20%? .

Citing Supreme court bJustice O`Connor:

"Others may copy the underlying facts from a publication but not the precise words use to present them" or "....facts contained in an another`s publication to aid in preparing a compiting work, so long as the compiting work does not feature the same selection and arrangement"

Seems to me that minor changes into the original expressions are enough to avoid infringing somebody else`s copyrightsm but I could be wrong, are there cases taht deal with this issue that i could study?

Thanks for any guidance.