I never use it. I don't find it helpful, one because of the huge ranges, two because it doesn't address the variability in lifts, three because it doesn't address the variability in lifters, four because it doesn't address the variability in goals for a given lift/workout/program/etc.
The optimal rep Rx is for the reps in the associated % range. So you would have needed 15 more reps between 70-80% to meet its optimal Rx. But in the case of your workout, the goal wasn't to accumulate volume at that %, and notice that you were doing singles, where the table would have you doing 3-6 reps. Doing 18 sets of 1 is much different than doing 6 sets of 3.
Now consider the % range - 70-80%. That's a huge spread. 18 reps at 70% is not a big deal. 18 reps at 80% is pretty tough.
It can be used best in my opinion to simply see if the volume of an exercise prescribed is reasonable. i.e. if you give someone 10 sets of 2 at 90%, you can check the table and see that's outside the range and you should probably knock the number of sets way down. But honestly, stuff like that is pretty obvious I think - not many people would prescribe that kind of volume w that kind of weight.
Then consider something like pulls, which are nearly universally prescribed based on max weights of the associated classic lift. 90% x 2 x 2 clean pulls isn't much of a pull workout, but according to the chart, that's optimal.
So what you end up with is needing to interpret the table pretty creatively, which to me means not really using the table and just using your own judgment anyway. Maybe I just don't get it. But I've not met any weightlifting coaches who have used it (or admitted they do at least), presumably for the reasons above.