I don’t really use percentages. For two main reasons:
Strength fluctuates not only day-to-day but even hour to hour (it normally peaks twice during the day, 3 and 11h after you wake up). So what represents 80% one day at a given time could actually be 85 or 75% on another day and time.
While there is an average rep number that can be done with a given percentage of your 1RM, it’s just an average. Some people can do a lot more reps than they should be able to do at certain percentages and others will do less. For example, at 85% the “average” maximal reps is 5. At my peak I was barely able to hit 3 or 4 reps at 85%. Not to mention that some of these percentages will change based on the lift.
The number of reps you can get at a certain percentage depends on your muscle fiber make-up (slow-twitch dominant will get more reps, fast-twitch dominant people, less) and on training experience. For example, people who train mostly with super low reps will suck at moderate and high reps and will have to use a lot less weight than the theoretical average. On the other hand, those who trained mostly with high reps will not be able to add a lot of weight as they decrease the reps.
Furthermore, because the hard 5s use very short rest intervals, the weight you can use will be impacted by your recovery capacity.
So I could tell you that in you should use 75% in reality it will work for around 70% of the population IF they are on a good or normal day (it could still be too heavy if they are not well rested, training in a suboptimal state, lacked food, etc.).
If you are dead set on using percentages all I can do is give you a range 70-80%.