I had this problem trying to work out how much strength i'd lost after some time off. Setting up programs on my old 1rm during a bulk wasn't working because i'd fail early and not hit the volume. At the same time my fuckarounditious meant i wasn't comfortable going heavy.

A good technique I found was just do 3 reps, take a rest, then add 5kg until I found/felt my limit or RPE was high. So I took bench for example, knowing my old 1rm was 127kg, I estimated my new rep back was about 110kg.

I stated at 60kg for 3 reps, just went up in 5kgs: 60/65/70/75/80/85/90/95

At 95 I felt great and decided to do 5 reps. I felt I couldnt another do 100kg for 3 for fatigue, but was pretty confident had I not done the extra 2, I would have been able to do 100kg for 3. So after accounting for the extra long warm up, I estimated my working one rep max to be about 110kg and based my programing off that. So far it's worked well. My programming is challenging, making me uncomfortable, but it isn't impossible.