The only difference between a skein and a ball is the way they are wrapped: the shape the yarn is wound in. It has absolutely no relationship whatsoever with the amount of yarn involved and so has no bearing at all on yarn amount calculations.
The best and most accurate measure for calculating amounts is yards, and all Lion Brand yarns give yardage for that reason.
If you are working from a pattern that doesn't give yardage, you will have to rely on the weight, but, in that case, far more margin for error needs to be left.
Because of different fibers and spinning techniques, there can be as much as 100 yards difference between 8 ounces of one kind of, say, worsted weight yarn, and another -- enough to really mess up a project.
When you think about it, when you knit or crochet, what's going through your fingers isn't how heavy it is but how long it is.
For example, one Pound of Love is 16 ounces, 1,020 yards. To use Fishermen's Wool in a pattern calling for one Pound of Love, one would look at the yards in Fishermen's Wool: 465 each. 1,020 divided by 465 = 2.19. Each Fishermen's Wool is 8 ounces, so you can see if you just bought 2 Fishermen's Wool (to equal 16 ounces), you might well come up short, depending on whether the designer used the entire amount of Pound of Love for her design.