If I hit on a 3, the Dragon Dice can show anything, the chance of getting a double is 44.44%, so on average it is 1.55 (3.5 * 0.44) Stunt Point per attack.
If I hit only on a 18, the Dragon Dice can only show 6. If I multiply it with the hit chance (1/216) it is 0.277 Stunt Point per attack.
How can I calculate the parts in between?
Theoretically, it is (Hit Chance) * (Double Chance) * (Possibe Stunt Die Size). I know how to get Hit Chance, but could you help me with the other two?
Summary on Stunt Points
For attacks (and other kinds of tests) you roll 3d6, 2 of these is generic, 1 special, from a different color, called Dragon Die. If at least 2 of the dice show the same number, you get Stunt Points equal to the number shown on the Dragon Die. It does not matter if the other two dice show the same number and the Dragon Die is different.
If you roll 4+5+6(DD), you most likely hit, but get no Stunt Points.
If you roll 3+4+4(DD), you get 4 SPs if you hit.
If you roll 2(DD)+6+6, you get 2 SPs if you hit.