For every generation back, the percentages are cut in half. So if a sire and dam contribute 50% each, grand sire's and grandams are 25%. Third generation is 12%. Fourth generation is 6%. Fifth generation is 3%, and sixth generation is 1.5%.
So if say a horse was in the 8th, he/she would have to have quite a bit of linebreeding to get the percentages up to make much of an impact. Especially if there was nothing bringing it forward.