if done properly with legs extra wide and sitting with your hips so far back that your knees are above or even behind your feet it becomes heavily dominate on working the hamstrings and glutes. louie simmons calls box squats a standing leg curl even. due to the mechanical differences i would recommend starting with 80-90% of your squat max.
but then i only do box squats right now for speed work and have only gone up to 60% of my squat max. i am also watching my regular squat climb every month after 3 months of doing box squats every 4th workout with 45-60% of my squat max. that is the only change i made since last year where my squat stayed stagnate nearly all year. i have already added more in 3 months then i did all last year.
i would also drop the leg press as your warm up exercise and focus on warming up using squats and working using squats. if you want more for legs after that, especially after box squats, do some unilateral leg press or even some step ups or if you can belt squats or front squats.