Squats - Pelvic Tilt Position?

Hey all, I've been trying to get good solid control over my Single Leg Squat...I've been doing negatives on the way down with two-legged squats on the way up. One thing I noticed, when I tilt my pelvis so that my lower back is flat (rather than arched), the movement becomes much, much more difficult. If I slow down the pace to do it 7-1-7, I can barely do 10 squats with BOTH legs. But one thing I notice is that with the pelvic tilt, I'm really starting to work that ROM where the quads cross parallel to the ground, and my torso is much better at staying upright. Also I notice that my knee movement becomes minimized, so I can really focus on form.

Should I focus on a flat back/hollowed torso position? Or should go back to letting it arch?

Thanks

At the bottom of a squat the back should be arched. I'm not a coach or anything. But everything I have read or seen on squat form recommends an arch at the bottom. Saying you let it arch at the bottom seems strange as most people have to try to maintain the arch at the bottom. I think the arch stretches the glutes and hamstrings at the bottom.

With single leg squats the back looses the arch at the bottom, in every photo or video I have seen of one. I don't know if that's ok.