there are several techniques for 'guestimating' the tail length.
1--(best i think) determine width to cast on (cast on 20 inches? or 30? or 60 inches? you tail needs to be 3 times width, + 20% more.
20 inches (sweater back say) =60 inches of tail + another 12 inches (10% of 60 is 6 inches, 20% is 12 inches.) so total? 60 +12=72.
It doesn't matter what size yarn or needles. (you'll might cast on 100 stitches at 5 stitches to inch, (sports weight yarn) or 40 stitches (at 2 stitches =1 inch--super bulky yarn)
the super bulky stitches eat up length--the finer stitches have a greater number.. but both require about the same length tail (3 X width +20%!)
(the reason? the tail is a 'stitch' (its formed with thumb) and as a general rule a stitch is a loop (a misshaped circle) an you need to have the radius (Pi) for the length of each stitch..
(so 20 inches of stitch need 20 inches X Pi (or 3.14 (or 3 times +20% (or 3.2) inches of yarn!) (you can use Pi if you have a scientific calculator handy.. but... most of us don't!)
(you can also wrap yarn round the needle (oh, wrap it round the needle 10 times or so, (and then multiple that length to get length for 60 stitches (6 times length of 10 wraps)
when you wrap around the needle, you are measuring the circumferned (get it? circumference--circumference is also found by measuring diameter X pi.. mmm, a Pi is a common to both of the methods!)
or you can use the 2 ball of yarn method (use 2 separate balls of yarn (or both ends of center pull ball (ie inside and outside)
tie them together, making a double yarn slip stitch (for this method ONLY--don't count slip stitch as a stitch)
then continue using one yarn (outer) as thumb yarn, and other yarn (center pull) as index.
when done, cut thumb yarn, and continue with index finger yarn.
(down side? 3 tails to weave in..
Upside--you never run out of tail! (even with crazy long (300+ ) stitch cast ons
(discard double slip knot (before closing into round, or at end of first row--and NO, it won't undo or weaken the cast on!)
OR--learn some other cast ons! there are lots of different cast ons.
but to be honest, i know lots of casts ons, (40+) and still use long tail (or a variation of long tail) 50% of the time!