Split squat form help!

I want to do Bulgarian split squats as part of my leg day routine. I work out at home, and don’t have an exercise bench. I have a folding chair I set up to use for this but I think it’s too high… I’ve watched so many videos on how to set up / proper form and they say to use something about knee height. The chair exactly that on me (I’m 5’2”).

I video myself doing it to make sure my form is right and I can’t seem to get it down. My knee seems to always go over my toes. Like I’m leaning too far forward maybe? But it also hurts my back foot, and feels like it’s just up way too high. Do I just need to keep trying harder on my form or is the chair I’m using maybe just too high? Does the back foot need to be high to make the squats effective? Is there something else other ppl use at home in place of chairs or bench for these?