How to choose a Weight for give an exercise?

This was the clearest explanation I’ve seen on this yet. Thank you!

Great! One thing that I noticed in my training is that sometimes when I pick a range for my reps, I reach to the failure (At least I feel that) right at the end of the last rep (e.g 8th reps). However, Sometimes I tell myself what if I pick a range of 10 with the same weight, surprisingly sometimes I get to that rep 10 too. That being said, I also figured I unconsciously increase the pace of my working set to reach to the goal of 10 rather than 10. Has that ever happened to you during all these years? Sometimes I feel like picking a weight and focusing on the form without counting might help me to more wisely pick my weights, and then based off of that I move to a next step which is counting my reps. Do you think this would work?