I hesitate to enter into this exchange, but I want to remark anyway that the waters are deeper here than they look.
First, there is the question of percentages. Lou is right that the advantage of 9-8 is 11.1 % and that the advantage of 8-7 is 12.5 %. So the difference in absolute advantage between the two spots is a measly 1.4 %. But to compare the two spots you have, it seems to me, to divide that 11.1 % advantage of 9-8 by the 12.5% advantage of 8-7. That comes out to just about 89%, which means that the player getting 8-7 has 11% the better of it than the player getting 9-8. (11% is the difference between 89% and 100%.) That’s one reason why, as Jay recollects, Ronny Allen could offer Eddie Kelley 9-8 on his break but not 8-7.
Second, I’m not at all sure that calculating a spot is a purely mathematical exercise. Elements of the actual play of the game must play a role, and I think the determining feature there is how many turns it takes the weaker player to get to whatever his required number of balls is. I play a player who will give me 9-7 but will not stand for 9-6. (Hey! I never said I could play well!) The difference lies not so much in the percentage difference between the two spots as it lies in the fact that if he sells out twice at 9-6 I am likely to get out, but I am much less likely to get out at 9-7 if he sells out twice. I probably need a third mistake from him to get to 7.
With larger spots this factor of the play of the game can become even more significant for calculating the percentage value of a spot. Suppose a top player is playing an amateur 15-5. Apparently the lesser player has a 66% advantage, and can probably get out in two or three chances at the table. But we all know that in reality this spot is much more difficult for the player getting the handicap than arithmetic by itself would make it appear. Part of the reason must be that the balls that that the better player has to spot come right back into play when they are put back on the table. They don’t go up table. They come right back into the action area where the better player can more easily pocket them. So a purely mathematical calculation of the spot would contain what is sometimes called “a conceptual error.”