Start with 85%. Train. Check progress. Make notes in your training journal. Assess.
Good? Then stay with it. Bad? Drop to 80% or jump to 90%. Train. Check progress via training journal. Assess.
In general, each lift will have a sweet spot, as mentioned in the Forever book. That is why you constantly use programs to assess each and every lift/program. We have people on the forum that have done this and have accurately figured out each lift simply by assessing and keeping detailed training journal.
The only question is if you want to do the work required to use your experience.