First and most importantly, please, please read the
original thread if you haven't before. Hell, you probably should read it even if you participated in it, since we've all probably forgotten quite a bit.
Secondly, if someone's data seems a mile off compared to where you would expect it to be, it's a good idea to verify it. For example, when Kassel says he hit for min 49% of the time with his new Ragebringer, and you recall that you were hitting for min LESS THAN THAT WITH YOUR FUCKING 10 DMG SEBILITE CROAKING DIRK, maybe he got the maths wrong.
With assassinations removed (872 BS):
http://pastebin.com/QE3n9dMN
Min hits (222 BS):
http://pastebin.com/4mAESEnJ
222/872 = 25.4%
Regarding the max hit. You can't hit for max with just max STR. Remember that your character's max multiplier roll is calculated by STR + WPN_SKILL + ATK. Max STR w/ Ragebringer gives you 255 + 225 + 40 = 520. The level 60 rogue damage table's max multiplier roll of 570 is what gets you to that final 553 max backstab. So you need an additional 50 ATK from somewhere to even have a slim chance of hitting the max. If you have avatar on, then your character's max multiplier roll is 255 + 225 + 140 = 620. Since it clamps at 570, any roll over 570 is going to give you the max multiplier for that attack, so you'll have a (620 - 570)/620 = 8% chance for max multiplier each attack. But beyond your character's multiplier roll, there's also factoring in the ATK vs AC roll. You also need to roll a d20 on that in order to see the final 553. If you really want to see a 553, you should have somewhere around a 7% chance to land one on a successful hit if you have avatar and are fighting low greens.