Think of RPE as a guide, not the focus. For example, in my current programming I work up to three ramping sets of 4 @RPE 7, 8, and 9 respectively. RPE 7 means I have about three reps left in the tank, RPE 8 means about two, and RPE 9 means I probably had one rep left. I then do backoff sets of 4 at a little over 90% of my top set with no more than 6 min rest in between, until the last rep of a given set feels like a 9 (typically the second backoff set, sometimes a third).
I probably couldn't give the precise difference between a 6.5, a 7, a 7.5, or even an 8. However, if I am on an RPE 7 set and am not sure I had a rep left, that means either I'm having a shit day or I overshot. Similarly, if I'm on an RPE 9 set and it's easy, I probably undershot.
As a practical matter, I just look to increase at least the top set (RPE 9) by 5 lbs a week (5-10 lbs/week for the deadlift), and the other numbers (RPE 7 and 8) get adjusted to account for the increase at the top...obviously the higher my top set, the higher the sets below it will be. As a general rule, this usually puts my top set around the RPE 9, and the sets lower than it at around RPE 7 and 8.
For example, I might do 345x4 (RPE 7), 375x4 (RPE 8), and 400x4 (RPE 9) one week. The next week, the top set will increase to 405x4, and then I'll adjust the lower weights to account for the increase...I might increase them (say, 350x4, 380x4), I might increase only one of them (say, make the RPE 8 a 380x4 but leave the first one as is), or I might leave the lower two around the same (unlikely, but I've done it when I believed I overshot the lower sets the previous week). I might even leave the top set as is if the previous top set was really, really difficult (for example, if it was a maximal effort grind with nothing left afterwards).
As I said...not an exact science, but it's a pretty decent guide. It also gives you a mechanism to account for bad days where things are just too grueling...if you're having a bad day, then reduce the weight to around the proper RPEs and just get through the session as best you can.
One thing I'll say for it...it's a lot more effective if you actually test yourself to failure once in a while. For example, sometimes that third rep just felt terrible and I didn't think I had one more...but I don't know unless I try for it. After failing several lifts, I gained a pretty good idea of when I'm completely spent, and when I can bang out another one or two reps. So if you're going to dive into RPE but don't know a lot about your maximum capacity, force yourself to try to complete the reps even if you think you can't, just so you can get a better feel for what you have left.