Sometimes you see a great-looking yarn but the brand is unfamiliar and you're not certain whether you'll get the garment you'd like. You don't want to be short of what you need, but if it's a great yarn you don't want to skimp and then have some left over. The answer could well be to knit from the top down, especially if you're using a plain old stocking stitch, tweed stitch, garter stitch, etc. Barbara Walker has recently re-published 'Knitting from the Top' as a paperback - Amazon has it - but any good generic pattern will do. Also, at some point you can get into circular knitting if you hate to purl.
I make a long crochet chain on a big hook with non-furry waste yarn and pick up the stitches for the back; work down to the armholes - just reverse the shaping - and then undo the chain for one side of the neck and pick up the stitches using your working yarn. (You'll be half a stitch out, which is why this works best on plain fabrics). Work to the neckline, leave, do the other side, cast on across the neckline, work down to the armholes, and join it all with a circular needle. At some point you'll get an idea of how long the body can be, whether you're making a slipover or short sleeves or long sleeves ... and if you're running really short you can do the welts, neckline, etc. in a contrast yarn.
I first did this with some multicolour yarn where I wanted to use every last inch of it and would really have resented the waste, but now it's my preferred method for almost any garment where the fabric is suitable. You can also do the fiddly bits (like neckline finishing) part-way through instead of having them all left for last. And you could knit your extra-precious yarn together with a coordinating plain yarn for the body and sleeves and then use the plain yarn for the welts, which will be finer - gives a more interesting garment.