Insomnia strikes again. I'm going to convert the dice pools of Mansions of Madness into percentiles so you can convert Call of Cthulhu scenarios into Mansions of Madness scenarios.
Use the following to convert difficulty ratings to dice successes on the Mansions of Madness D8s which have a 2/8 chance of 1 success (not using clues)
25% 1 Die
44% 2 Dice
58% 3
68% 4
76% 5
82% 6
87% 7
90% 8
92% 9
94% 10
96% 11
97% 12
98% 13
99% 15
100% 19
Note that this is only the probability of 1 success.
For rolls requiring 2 successes, use the following chart to convert Call of Cthulhu percentiles into Mansions of Madness dice pools.
0% 1 Die (impossible because you can't roll two successes on a single die)
22% 2 Dice
29% 3
34% 4
38% 5
41% 6
43% 7
45% 8
46% 9
47% 10
48% 11
49% 13
50% 17
For 3 successes...
0% 1-2
19% 3 Dice
23% 4
25% 5
27% 6
29% 7
30% 8
31% 10
32% 11
33% 13
The math I'm using is (1-(6/8)^X)/Y
X = the number of dice
Y = the required successes
If my math is off, meh.
The Mansions of Madness dice are gimmick dice that have 8 sides but only 3 outcomes. 4/8 mean failure, 2/8 mean success, and 2/8 are failures unless you spend a "clue" token to make them a success.
You can just use d4s where 1-2 equal failure, 4 equals success and a 3 can be turned into a 4 with a "clue".
1 success for common tasks (50%+ with 3 dice). 2 successes for challenging tasks (33% with 5 dice). 3 successes for barely possible tasks (25% chance with 5 dice)