I'm awful at judging RPE, too, don't feel bad. I was doing 6x3 bench last week and was supposed to be RPE 7-8 for the sets and the sixth set was an AMRAP. I could've sworn every set was a 9, then got 7 reps on the AMRAP. I think part of my problem might be not warming up enough, too.
I don't know if you've read The RTS Manual or much on their website, but this chart might help you a bit.
That's RPE on the left, reps on the top and percentages of 1RM in the middle. It's just a generic chart and they actually recommend everyone come up with their own over time and through experience as we're all going to be a little different. It's a place to start though.
They also recommend that after your warm-up sets you do a set at 90% of your projected top set (same reps) and then 95% (also same reps) and then make adjustments to your top set based on that. Go up if you feel great, down if you don't or just stay the course.
Anyway, I hope that helps.