I have a casting on trick that I learned from Gudrun Johnston that works great with a long tail cast on when you don't want to have too much yarn left over. (It could also work with other cast ons, but long tail is where I typically use it.)
Cast on with a fairly short tail - enough to cast on about 10 stitches and have a long enough tail for weaving in at the end. Then, unravel the 10 stitches you cast on, making sure to keep one hand on each end of the yarn that was used in the cast on (so, one hand on the start of the yarn tail and one hand near the first loop) - the yarn between your hands is the amount needed to cast on 10 stitches, so you can use that to measure off how much yarn you will need to cast on all stitches for your pattern, plus allowing for the weaving in tail, and you know where to make the slip knot for your first stitch.
I hope that makes sense - it probably makes more sense visually than in words, but it does work.