Several days ago I came across an interesting thread at http://www.knittingparadise.com/ discussing this problem. The knitters suggested quite a few ways of calculating the length of yarn tail BEFORE starting to cast on.
Here's a summary of what was suggested:
1. The length of tail is 1 inch per 1 stitch. That is, if you need to cast on 50 stitches, the tail should be 50 inches. Should work fine for worsted weight yarn.
2. Another lady suggested to wind yarn loosely around needle so many times as the number of stitches you need, then to let the loops go - that's the tail.
3. Another way to estimate is to leave tail 3 times longer than finished knit. For example, if you cast on for a back of sweater 50 cm (20") wide, the tail should be 50x3=150 cm (20"x3=60").
4. You can also cast on 10 stitches, then take them off and measure the length of tail you needed for 10 stitches. Then divide the length you measured by 10 and multiply by the number of stitches you need.
5. A quick way to estimate the length of yarn tail is to measure the length of your arm. One of the knitters participating in the discussion said that arm's length of yarn should be enough for casting on 30 stitches in worsted weight yarn.
|Picture from http://www.knittingdaily.com/|
# of stitches x 1.1 + 15 = length of tail in cm
e.g. If you need to cast on 50 stitches, the tail should be 50x1.1+15=70 cm long.
I mostly work in sport and worsted weight yarn and this formula works just fine for me.