Don't think there is a set benchmark figure. If your goal is to get to a 100kg 1rm, and you're doing 3x8 to get there, then as you increase the weight steadily you play it by ear as you get closer, play around with the reps and you'll have a fair idea how close you are.
Using your example of 3x8 @ 80kg, if you get to that point then you might want to try a set with 5 reps at say 90. If you did that fairly easily, then you'd know you're pretty close. When you think you're close just give it a shot and see how you go. I'm no expert because I can't do it yet, but just saying that's the way I'd approach it.