Can't keep back flat on deadlift - 365x4x1 FAIL

[Jack Morrison]

I have struggled with this for a long time. What cue should I use? How much weight should I take off the bar? Every time I fail a rep, it looks like this.

YouTube

[Pete Troupos]

Jack, Jack, Jack. We want to see the setup, but a full minute before pulling the first rep? Please trim it next time.

You're pulling from your heels and your first move on a lot of these is to open the hips to get the bar off the ground.
Setup with you the bar across your straps and then bring your shins to the bar by sitting your hips down a bit and bending the knees. Make sure you're midfoot, then set the back, and drive your feet into the ground to get it going.
As for your back, you're just not getting all of the slack out of the bar and setting very hard to begin with. That "click" you hear when the plates come off the ground should be heard when you're setting the back before you even think about starting. Lifting the chest to get the back set hard should pull the arms taught and cause the bar to bend upward. You'll "feel" most of the weight in your hands when you do this right. See if you can get the bar to come off the ground silently. Loosen your belt one hole.

Work on those things for now. There are a couple videos on the youtube channel that may help if you search "deadlift" in the videos, including one on taking the slack out of the bar.