A lot of good advice here, but it's mostly saying what not to do, rather than how to actually do it [chamber against good players].
Before you even you think about seriously chamber blocking in a duel, you need to bump up that percentage. Get it up to 95%, preferably 99% success against bots.
Contrary to popular belief, it's actually very possible to [attempt to] chamber every attack. This is how I fight, and I wouldn't really recommend it. But, if you really want to know how to do this without dying -- you will need to die. A lot. Like a ****ton. Slowly but surely you'll develop a sort of sixth sense w/r/t when people will hold their attacks. This isn't to alert you when not to chamber -- you will be attempting to chamber everything -- it's simply a reflex to make you cancel that attempted chamber. Of course, there are times when it would be physically/electronically? impossible to start a chamber in time-- you'll need to die to develop the sense for that as well.
Eventually you'll get to the point where you can chamber like nobody's business. And it'll be pretty cool man. One caveat though: chamber blocking against good players does absolutely
nothing (excluding stab chambers). So, it's satisfying and all to land them, but ultimately you'll realize it was a waste of time, and so you'll feel like this

for a while. But, total chamber mastery is priceless against average players, so after a bit you'll feel more of an empty

.
Examples of this [pointless] style:
This duel against Sarah
Ten chamber attempts here. Some are pretty short, would probably need slo-mo to catch them -- my first chamber attempt here lasts for like five frames. (Sixth-sense in action!). A couple other chamber/attempt-heavy duels are at 4:30 and 10:10. I counted 80 chambers/attempts throughout this whole video. Why I bothered to count, no idea. And there would be way more if Sarah had a different style -- tons of holds here, etc.
Basically chambering good players gets you killed and no kills. But, always fun to try, usually.