Here’s how to CO without having to estimate the tail length:
Use two tails of yarn, either from
two separate balls of yarn, OR
a tail from the inside and a tail from the outside of a center-pull ball
Leave at least 3" on each tail, tie the ends together with a slip-knot
Slide the slip-knot onto your needle and tighten it up
Now separate the two working strands as if they were one long strand and work long-tail CO as usual.
With left hand wrap yarn around finger and thumb (as seen in photo and video)
With right needle, come under thumb loop of left hand
Then come over finger loop and draw a loop of yarn from the finger loop through thumb loop
Release, tighten up that stitch, and reposition fingers/ hands for next cast-on stitch.
When you have all stitches CO (do NOT count the initial slip-knot), cut one of the working strands, leaving a tail that you can later work in.
Begin knitting with knitting with the un-cut strand of yarn and when you come to the other end of your CO, get rid of that slip-knot, just slide it off the needle and pull out the knot.
If you are casting on fewer stitches, check out the traditional method.