Minecraft wiki entry for Chorus_Flower, associated text: 100% chance if zero or one plant block is directly beneath
60% if the flower is above a two-block unbranched plant (i.e. two plant blocks over end stone)
50% if above a two-block high branch (i.e. two plant blocks over air)
40% if above a three-block unbranched plant
25% if above a three-block high branch
20% if above a four-block unbranched plant