Chair deadlifts help off the floor? I thought they were for the lock out because the legs' involvement is minimal? I ask because I was recently looking at these, wondering if I want to try them specifically to help with lock out?
Absolutely! Dimels will help your lockout, but chair deadlifts force you to wedge in correctly. So many guys try to just squat down to the bar and stand up. That puts the start of the lift like trying to push a car by getting a running start and slamming in to it...... wedge yourself in, pull the slack out of the bar and out of your body, then drive hard like you're coming off of a box squat...... chair deadlifts are just the first two steps to a box.
As for the advice to the OP about bands, chains, and speed work....... that's all great, but if you're missing at the floor it's a positioning and technique issue. The only way a strength issue is causing this is if you lack some fundamental strength to wedge in ( which I doubt).
Case in point - a few years ago I called for a 777 deadlift to break Ernie Lilliebridge Sr's record in the APF of 771...... I got a misload with 854 and still pulled it to the knee even though my contest best was 733.
ALSO - a buddy and I are both around 800 deadlifters. His contest best is just above 8 and mine is just below...... just for fun we both opened at 600 at a charity meet and when I went to wedge in to start the lift I pulled the plates 2" off the ground before I even drove my legs.
Learn to set up right, with your chest up and your knees/ankles vertical or close to it and you'll never miss at the floor.