How to apply it:
Basically, the above table by yours truly is saying that if you take your 1RM on a lift, you should (in arbitrary theory) by able to perform 95% of that same weight for 2 reps, 90% of it for 3 reps, etc. So if you know your 1RM, you can predict your 2-20RMs as well, using the following formula:
x = actual 1RM. y = percentage for target reps.
Therefore if your 1RM is 150kg, and you want to perform a 5RM (which, is 85%1RM), you would multiple 150kg (x) by 85 (y), and then divide the number by 100. 150x85/100 = 127.5kg.
Alternatively, if you want to predict your 1RM based on what you can do for reps, use the following forumla:
z = RM for reps.
Therefore, if your 5RM is 127.5kg, and you want to guestimate your 1RM, with your 5RM being 85%1RM you would divide 127.5kg (z) by (85 (y) divided by 100) (or, more neatly, you'd plug in 127.5/0.85). 127.5/0.85 = 150kg.
Why it's wrong:
Other than the high likelihood that since high school I've forgotten how to accurately write up an equation, this is all wrong because of that SAID principle:
That is, you get good at the stuff you do, more so than you get good at the stuff you don't do. This should be intuitively obvious, but once we discover that 1RM calculators exist, that intuitive obviousness has a tendency to go out the window in our minds. "But such-and-such-a strength and conditioning coach has a great calculator on his website for this stuff. It even has percentages like 91 and 87! Surely this stuff is scientific!" It may be scientific, but it's still wrong. Okay, "wrong" may not be the nicest word, but it applies. Research and experience allows coaches and sports scientists to test people's strength in different rep ranges, and conclude mathematical averages for how a 10RM relates to a 5RM and 1RM and so forth. But these are averages, not absolutes.
If your 1RM is 150kg, it does not guarantee that you'll be able to achieve 5 reps at 127.5kg. If your 5RM is 127.5kg, it does not guarantee that you'll be able to hit 150kg once. If you do all your training in the 1-3 rep range, then your 20RM will probably be below the guestimation on the table. If you spend all your training in the 10-20 rep range, then your 10 and 20RMs may actually be higher than 75 and 60%, respectively.
Besides that, people have different ratios of muscle fiber types, different techniques and different leverages, allowing different degrees of talent at different intensities and volumes.
And that's just dealing with relatively simple exercises like squats and bench press. Take a look at complicated, highly technical lifts such as the Olympic lifts, and these predictions become even further removed from reality.