Your back looks pretty flat, but I suspect you could get it tighter. Nipples to the wall is a good cue, but one of the things that really helped me was learning to use the weight of the bar to pull myself into the right position. The idea is to pull all the slack out of the bar with your arms as you set your back. Try this exercise. Put a relatively light weight on the bar and follow the 5 step setup procedure. When you get to step 4 (squeezing the chest up), squeeze up so hard the plates lift 1" off the ground. Then put it down (don't do a full deadlift). Do this five times and on the fifth one, after you squeeze up, do a complete deadlift. Do a few sets of that exercise with a light weight and then move on to your normal deadlift workout. If you do it right, you should start to feel a new level of back tightness.
Also, tuck your chin or you're going to hurt your neck.