I think a lot of that has to depend on the quality of the two goats you are breeding, two goats with the same fault should never be bred together, this holds the same for goats even not related. Line breeding intensifys good traits, but it also intensifys bad traits.
There was an artical in dairy goat journal last year on line breeding. It was really good, this is how they described a good line breeding, they described a good line breeding as one animal apearing in the pedigree some two to six generations back three times. So when laid out on paper it formed a triangle with that goats name.
There is a page on the ADGA website, where you can type in the potential sire number and potential dam number and it will calculate the top ten animals contributing to inbreeding and what the percentage is.
I like to keep a generation or even two in the least when line breeding. But thats my personal opinion. Some people will breed a doe back to her sire, half sister to half brother and so forth. I had a friend that bred a buck back to his dam she got a nice kid that did well in the ring as a kid, never did steller as a milker though, and a full sister that had three teats.
hope this helps and i didnt confuse you