If you're going to use a yarn other than the one suggested in the pattern, you're going to have to figure out how much you need. You certainly don't want to end up with dye lot issues!
The pattern calls for Cascade Sierra, which as 191 yards per hank: 4 hanks each for the small and medium and 5 hanks for the large. But it also gives estimated yardages for each size: 650, 800 and 950 for small, medium and large respectively. If you're using a worsted weight yarn that gives you the recommended gauge, AND if you're knitting one of these sizes with no modifications, you should buy that quantity.
But what if you want to lengthen the sweater, make a size larger than what's in the pattern, or make the sleeves short rather than 3/4? I'll admit that I don't have a perfect formula to figure that out. But, in general, on a high-hip length sweater with long sleeves, the sleeves make up about 1/3 of the total fabric. And we can use proportions to make an estimate of how much we need.
Let's take the size small as an example, and lengthen it. This size calls for 650 yards of worsted weight. Approximately one third of that will be for the sleeves, so let's say about 200 yards, leaving about 450 for the body. If you want to make the sweater longer, think about how much longer - maybe 1/4 of the length to the waist? So you'd want another 1/4 of the 450 yards, or about 115 extra yards of yarn.
Now, if you're changing the weight of the yarn, it's a different ball game. My first tactic is usually to cheat: I take a look at that Ravelry page again to see if anyone has made the sweater in the yarn I'm using! This can give me a great clue to how much I'll need. If not in the same yarn, has anyone made the sweater in the same weight yarn? I generally prefer to do detective work over math!
In any case, I think it's always a good idea to buy an extra ball or two of your yarn for any project. You never know - even yarns with the same dye lot can be noticeably different in color, like the project I linked to at the top! The good news is that with a top down sweater, you're making the whole sweater as you go along, so you have more leeway to change things as you go along. But knowing in advance what changes you plan to make will help you to purchase the right amount of yarn to get you there.
Well! I think we're about ready to start some actual knitting! Who has yarn and needles ready?