Best Workout Plan for Natural Lifters - ISO Pre fatigue question

Hey CT. Question on the isometric pre fatigue on the first rep. What should the difficulty be for that? Should it be I can’t hold that weight any longer than 10 seconds for the first week and then regular reps or should the hold be difficult enough where I can feel a really good contraction but could maybe hold it another 5 seconds if needed?

I’m trying to dial in the weights for that and not sure if I’m not going heavy enough since my reps were between 10 and 15.