I am currently farming the 76th floor for drops as I can do this one with my force on auto (They are all 6* lvl 99) and this is how my drop rate is fairing per 100 stamina runs:
(*stamina* - *Drop*)
100-0
100-1 (drop on last 12st)
100-1 (drop at 50st left
100-1 (drop at 85st left)
100-1 (drop at 24st left)
50-0 (No drop with 50st run)
100-3 (drop on first run at 100st) (drop at 38st left) (drop at 27st left)
100-0 (no drop)
So as you can see unless you have a lucky run you only really get one drop per 100 stamina at floor 76. Which makes this a hard event.