Yeah, you don't want to do 12 cm in one segment.
Although tbh I wouldn't really want to do more than 5 cm if I was doing 2 segments. 5 cm is the cut off point for the safest lengthening.
How does 173 cm sound to you?
Your arm span is pretty good, you won't need to worry about it. I'd advise against going more than 5 cm over it as a rule to most LLers, though (I think it begins to look noticeably short around that point).