[itimpi]

EDIT: I responded to a post without realising there were many replies already there!

Is that rule not another way of saying:
- make sure that a word contains at least 3 letters before the soft-hyphenation point.
- make sure that a word contains at least 3 letters after the soft-hyphenation point.
- do not soft-hyphenate words of less than 6 characters (implied by the above two rules)
If so one does not need to know the actual line length as the rules are part of the embedding soft-hyphens into words logic.