So far as I know, increases to damage from higher str or dex are linear -- there aren't breakpoints. When I did testing years ago on my hero, I was getting damage numbers for LW that scaled roughly as str+200.
See my post here for details:
http://vnboards.ign.com/hibernia_guardian_professions/b20907/97914021/r97938442/
-----signature-----
Alb Tristan <Wraiths of Albion>
Asilithiel, Speakssoftly, Kegnor, Kesteral, Badgers, Amarynthia
Hib Galahad <Juggernaut>
Hulainn, Hansil, Cythrael
"Friars are in great need of a donkey. We need something to carry our ale."