Crushthrough is mostly related to weapon weight versus shield weight. So against a steel shield (12.5 weight) don't expect it to always work even with +3 long maul (8.2 weight) or even the huscarl (9 weight)
I'm curently at 18 str (with rank 0 long maul for now, still looking for a seller) and i dont crush as much as i'd like, a fail crush often means death. With my last gen with long maul i had 24/15 (8ps) and i could crush through EVERYTHING everytime unless my swing was stop close to me. So i guess damage or strengh matters ...
I had a lot of fun with a 24/15 build, i could two shot everything and one hit was a 50% knockdown. I really loved how people under my swing could'nt do anything but die in 2 sec, and off course event a steel shield was no help at all.
I'm gonna try a 21/18 to see if the aditional footwork and swing speed can beat the 24/15 build.
My armor weight will be :
Head: 2.3 (Nordic Conical Helmet)
Body : 12.8 (Lordly Light Kuyak)
Hand : 0.8 (Scale Gauntlets)
Leg: 1.5 (Mail Chausses)
With an effective wpf of 130 (The last patch lowered the weight of every armors)
Hopping that with 7 PS and a mighty long maul i will still be that heavy crush machine which two shot everything.
I also always carry a long dagger to chasse archers (holding the long maul slow you too much for that).