to seam? Like you need X inches of yarn to sew up Y length of seam in Z weight yarn? It’s a pain to pull more than you need through, yet you don’t want to be short and have to weave in more ends than necessary.
Hmm…I just usually leave about double the length of whatever I’m seaming. It’s worked so far, though I do end up having a bit more left over than I’d like, even after weaving in ends. You’re right that it is a PITA to pull all that yarn through.
I tend to overdo it, too. Sometimes the yarn gets pretty ratty toward the end.