These evolution numbers are based on Rage of Bahamut's evolution, but the way it works should be about the same.

What it means, is basically... for example,

in this case of "4-1: 86.629%", left side "4" represents the number of cards used to reach final form of evolution, right side "1" represents the number of times the card was maxed (in this case, it would be when it was fully evolved" and "86.629%" would mean that the max atk/def you reached when you evolved 4 cards without maxing any of them beforehand is actually 86.629% of the card's full potential/full max atk/def that it can reach.

8-1 would mean the same thing, basically like,

(R+R)+(R+R)=(R+)+(R+)=(R++) - 4 cards used here

(R+R)+(R+R)=(R+)+(R+)=(R++) - 4 more cards used here

(R++)+(R++)=HR (which is then, used as target for sacrifice to reach full potential)

in other words, 8-15 would mean:

(maxed R + maxed R) + (maxed R + maxed R) -> (maxed R+) + (maxed R+) -> (R++) - cards are maxed 6 times here, in total

(maxed R + maxed R) + (maxed R + maxed R) -> (maxed R+) + (maxed R+) -> (R++) - cards are maxed 6 more times here, 12 total

then finally,

(maxed R++) + (maxed R++) -> maxed HR - cards are maxed 3 times here, making it 15 times total. 8 cards used, therefore "8-15", and the final maxed atk/def is the 100% potential of the card.

I hope this explanation helps.