Do I sorta have it?
Not sorta. You have it exactly. I "work up" to the RM. The 295 felt very heavy this session. I wasn't sure I could lift higher never mind with good dorm. 275 is convenient, just pull off the small plates, and still within the 90% of the RM. You get the target number of reps (7-8 in this case), but still keep them challenging, so you don't just go down to 90% and call it good enough. So the 7th felt pretty easy, so I went a little higher on the last rep.
I basically make the decision as to what I'm doing next during or right after the previous rep. It's the old "thinking on your feet" thing. Hard at times. Sometimes it would be a lot easier to just have prescribed reps and weights to follow.
The "working up" is more than just warm-up, but "warm-up" is the term that is most familiar to most people that is near the concept. It's more a matter of acclimation, getting used to heavy weights near where you intend to do your work sets. Plus, as I go up I have an idea about where the RM will end up, and any lifts that I do within 90% of the RM count on the target total, even if I lift them before the RM. A couple of weeks ago I was going up by larger increments, and jumped to 310, when the right RM probably should have been 300 or 305. I had some back pain, and some struggles finishing the workout. The small intervals give me the chance to "feel" and anticipate about what the RM will be.