Been using knockdown weapons almost exclusively as of late. I probably have somewhere around 25% knockdown success. Been using a Mighty LHSM, Bar Mace, Mighty LIM, Great Maul, and a Mighty Mallet. While the first part of the formula can be somewhat hard to max out, it's not hard to at least get 3/4 of the max weight bonus. The second part though, only requires you to do a minimum of 40 raw damage to max the bonus here, so even with a 3/4 weight bonus you're looking at a 22% chance to knockdown as I rarely have lower than 40 points of raw damage on a hit with these weapons.
Knockdown formula needs reworked so that the max chance is 20%, not 30 and you should need 100 raw damage to cap the raw damage bonus. I'd say just up the static number that gets subtracted from your raw damage to 60, but this will make it very hard to knockdown with 1hers.