I wish I could explain clearly the very simplistic method I use, but maybe this will give you an insight. Crossed Fingers
I determine how much yarn it takes for 10 sts and use that measurement to calculate how much yarn I need to cast on my required number of sts, plus enough to weave in (or even seam).
I simply hold the length of yarn against something for length and count it off by 10s 'til I get to my number.
Let me know if this makes any sense. :??