I told you that it doesn't matter. He had no affiliation with Liu Bei whatsoever. In the novel, he stated that Liu Bei would be able to rule the land instead of Cao Cao, which was probably to upset him. That's all he has to do with Liu Bei.
If you want, you can give him a value of 50. It would put him right between Cao Cao and Liu Bei, both showing his historical affiliation with Cao Cao but also some sort of ideological closeness to Liu Bei.
Cao Pi and Cao Zhi for example hate each other as well, that doesn't have any effect on their compatibility values though.
Zuo Ci, historically speaking, was more or less a subject of Wei for a brief time and did not serve any other force, so the logical thing to do is to give him a value that's closer to Wei than others. That's the benefit he has over guys like Hua Tuo, who didn't serve anyone at all in game terms. He can dislike Cao Cao all he wants, that'll probably prevent him from joining Cao Cao anyways, while him liking Liu Bei will have him start out with high loyalty should he join Liu Bei.