Ah right. I use 10 as a complete max. 9.5 as not having an extra rep but having more weight on the bar in me (where most of my training is). 9 as an extra rep, 8 as 2-3 extra reps. Everything below that is 7. I find I can judge this pretty well. When you look at it this way it's pretty objective. If I am doing heavy weight then obviously I want to start at a 9 and keep on doing sets until I do a 10. If I am doing volume then I start it at 8 and keep doing sets until I hit a 9.5. I use it simply as a way of determining how many sets I do, instead of picking a random, baseless amount of sets prior to lifting. No real need to make it more complicated than that!