I’ve heard a lot about knee pressure, and played the #1 dan on xbl who used knees a lot, but I cant find any good info on how knee pressure works and how to use it.
As far as exact frames, I don’t believe its well documented because its character, stand, crouch, and range specific. According to the Brady games guide, Knee can at max only be up to +3, but I’m almost positive that’s not the case. You might be able to get some loose ideas with FA-Tool using the cancel frame trap tool, but take whatever you get with a grain of salt.
The Knee of Justice, if it connects on its first active frame, leaves Dan at even frames with his opponent. However, it rarely connects on its first active frame, as that hitbox is higher up and most people will crouch-block, especially against Dan as he has no overhead attacks. This means that the Knee will hit on later frames… sometimes much later.
If the Knee connects on the last of its active frames, which requires very good spacing or timing, it will give you +10 frame advantage, an absurdly large amount for anyone, something comparable to Hugo’s claps. Usually it winds up being somewhere in between the 0 and +10, depending on exactly how late it hits. That makes the close MP a great followup in most cases, and back HK good for the very late hits, if you’re trying to frame trap opponents. Alternately, for those very late hits, you can just keep going for close MP as an actual blockstring into another Knee, which after a couple of repetitions is sure to start people mashing reversals to try to escape. Tick throws are great once you can master the timing on them, which isn’t easy because of how much the frame advantage can vary, but for people who are conditioned to wisely just keep blocking once the Knees come and eat the minor chip, the threat of a throw will force them to consider other, riskier options.
While it worked better in the previous editions, against people who smarten up and start stand-blocking your Knees to reduce your frame advantage, you could try a c.LK xx Koryuken FADC to punish them for their intelligence. Now, of course, with the uppercut FADC being punishable on block, that’s not something to just pull out without a confident read.
Thanks thats what I was looking for. I played dan in a tourny friday and my knee flew right over the top of a ducking honda tho! 
I assume there are a list of low box characters it wont work on?
A lot of characters can low profile it with stuff like a low forward or slide, other characters it’ll miss if used point blank. It cannot hit crouching Elena at all.
Honda, Blanka and Elena are the most egregious cases of low hitboxes messing with the Knee. Certain angles might still hit Honda and Blanka, as Ultra was supposed to fix the hitbox of the move to let it hit crouching foes better, but Elena is completely immune to it while crouching.
This isn’t unusual, because a crouching Elena is completely immune to most of Dan’s jumping attacks too. HK, MK, HP…
Guy is a bitch too regarding his hitbox.