I've never understood the practical uses of something like that. Please explain. If you want to know your max, why wouldn't you just max and find out for sure?
No - I can't remeber where I found it, just saw it on my HD and thought cool!
it's useful for estimating a weight to use for a certain rep bracket for example - or predict how much weight you need to be able to hit for a certain number of reps before you are capable of doing a certain max and so on