I was bored so I thought I'd make a little chart illustrating Kruciels point.
Boss Chance on Death of Boss:
                            15%    25% 40% 50% 60% 66.6% 75% 85% 90%
CT                        100%----------
Stone Monster       15       25    40   50      60   66.6  75     85      90
Insane Goblin        2.25    6.25  16   25     36    44     56     72      81
Black Unicorn       .3375   1.56  6.4  12.5  21.6 29.5  42     61      73
Chilled Devil         .0506   .39    2.56 6.3   13    19.7  31.6  52      65
Arcing Demon      .0076   .098  1      3.13  7.8   13    23.7  44      59
Mistress of Flame  .0011   .024  .41   1.5    4.7   8.7   17.8  37.7   53
Angryface            .00017.006  .16    .75    2.8   5.8  13.3   32     48

With people getting up to 5 CT triggers in a single pull in the zone I'm not sure what the 'right' number is, but 15% each is definitely low. 66.6% is the most "Flavorful" for halloween but that is definitely way too high.

It's a little tough to properly balance getting a reasonable amount of end triggers without making tons of stone monster, insane goblin etc spawns.. so its possible to flatten the curve with increasing percents:

15-25-35-45-55-65 would look like
15%
3.75
1.31
.59
.32
.21%

20-30-40-50-60-70
20%
6
2.4
1.2
.72
.5%

25-35-45-55-65-75
25%
8.75
3.93
2.17
1.4
1.05%

30-40-50-60-70-80
30%
12%
6
3.6
2.52
2.016%

Just some data to look at and think about
Zexan