Calculating 1RM across multiple sets?

So I use an app for tracking my lifts and it uses that data to show me a few different graphs, including my estimated 1RM. However, this calculated 1rm is based on each individual set, not on all sets - for example I did a set of 4 at 60kg for OHP, but that was my last set, I did 2 more at the same weight for 5 reps each before that. So, yes, it might be accurate to say on the last set, my 1rm was around 65kg, that says nothing about what my 1rm is while not fatigued.

At the moment, I'm just curious what the numbers would be, but I know some routines depend on your 1rm to determine how much you should lift and I'd like to try those at some point. Is this something that is just going to waste a workout session in order to figure out?