yo, yesterday i got pebble on floor 5 lol, and i managed to do everything without losing a heart. (I was with people, but we were stealthy and only a few died)
I've found a chart showing the chances, and this is what I've found: Floor 5-9 = 3.6% | 10-14 = 6.9% | 15-19 = 10% | 20-24 = 12.9% | 25-29 = 16.1%. [I Hope This helped!]