Let us use simple math and probability.
The standard starting doom pool is 2d6. So for starters we get difficulty < 4 66% of the time with d6 effect. That is pretty easy and not really challenging to most heroes.
So what would challenge the heroes? Let's say that a difficulty that they can overcome only 30% of the time is really challenging. How many doom dice would we need for this?
Boring calculations block start.
Let's assume Captain America as one of our heroes, and try to calculate his average success difficulty. (This is an difficulty that he can best 70% of time without the use of power points.)
- Average affiliation dice (standard for all heroes) is (d6 + d8 + d10) / 3 = d8
- Average distinction dice (which, again, is standard for all heroes) 3d8/3 = d8
- Cap has 2 power sets with average dice [(d12 + d8) / 2] = d10 and 4d8/4 = d8.
- Average specialty dice are [( 4d8 + d10 )] / 5 = 8.4, which is closer to d8.
Averaging specialties is a bit tricky because d8 and d10 can be split into smaller dice automatically, so let's assume now that there is no dice splitting involved.
In the end, Captain America has, for his action, an average die pool of 4d8 + d10. (This is completely bogus, but still helps us analyze difficulty.)
Let's assume that the d10 will always go to effect. In 75% of dice rolls total will be 11+, but only 35% will get 13+. Logically, if we want the Captain to fail hard we need our effect at d10 and a total more than 13, or an effect at d12 and total 10 or more. This gives us our "dooming" doom pool expectations.
Doom Pool Calculation
Reminder and clarification: Total is sum of two dice. Not sum of all.
So we need at least 1d12 and 1d10 to use as effect dice and a couple of lesser ones to give total of 13 or more.
Total of 13 or more with probability of 0.7 can be achieved with these dice combinations:
- 6d8. Doom pool consists of 1d12, 1d10, 6d8 - 8 dice.
- 5d8 and 4d6. Doom pool consists of 1d12, 1d10, 5d8, 4d6 - 11 dice.
So this is, pretty much, the doom pool that you really need to put good pressure on heroes and, via this, on players. (Moreso from a psychological standpoint: big groups of variable dice in the doom pool are more scary to players because a) there are a bunch of them; b) they are random and undecided, so this means more suspense.)
Doom Pool Building Strategy
- If you start with 2d6 in pool and heroes have more than 2 pp each, step up your current die.
- If players actively spend pp - start getting dice in the pool until you have 9. If more than one opportunity arises on the roll, buy a d6 and step up an already existing die.
- Stepping up higher dice is a more efficient way to build a high difficulty pool. Buying d6s in is a way to build a more consistent difficulty pool.
Also remember that stepping up does not give players pp and can stall the game with a high doom pool difficulty, so when you have a bunch (more than 2) of d12, start interventions -- which speed up the game, give XP to heroes, and drop difficulty at the same time!