Estimating yarn requirements (for seams, etc)

I’m a new knitter, and I’m about to seam up a hat, using kitchener stitch, is it? Any good tips/tricks on how much yarn to leave in order to do this? Although, chances are there won’t be much yarn left on my ball.

Or how about in other situations, like when they say, “knit until your yarn is gone, but leave enough for 2 rows,” or when you have an intarsia section with say, 24 stitches?

When it comes to estimating the amount of yarn–usually about twice the length of the seam works for me. However, if it’s a long seam (like a sweater sleeve for example), I usually use 2 or 3 lengths of yarn, even though it means more ends to weave. The reason for this is that it can weaken the yarn and make it look ratty when you sew with it for very long, and I also have trouble with the yarn getting tangled up when I have too much length.

As far as your last question, estimating is something you will get better with at time. I usually buy one more ball of yarn than the pattern calls for for reasons just as this, and because I tend to get paranoid about running out of yarn. I’d rather have some extra in my stash than run out!