The coffin jackpot is doubled if three or more physical balls are on the playfield, and subject to multi multiball multiplier. so you have to collect the coffin jackpot after the castle jackpot ejects the balls from the lock after collecting 3 coffin jackpots. if you have two trapped in the lock, you will only get a 3x instead of a 6x jackpot, and you need the 6x50 to get 300 million.
The Jackpot value depends on how many balls are in play. If only two balls are in play, the Jackpot value starts at 20M points and increases by 10M points each time it's scored, up to a maximum of 80M points. If three balls are in play, the Jackpot value starts at 40M points and increases by 20M points each time it's scored, up to a maximum of 100M points. The Jackpot value carries over throughout the game. The Multi Multi-Ball Multiplier is also applied to Coffin Multi-Ball. If Coffin Multi-Ball and one other Multi-Ball mode is active, a 2X Multiplier is applied to the Coffin Jackpot value. If Coffin Multi-Ball and two other Multi-Ball modes are active, a 3X Multiplier is applied to the Coffin Jackpot value.
As I said, the jackpot value was always what I expected it to be and I always had balls locked in the castle lock. Perhaps there was a bug they fixed that you encountered? I also don't think it would be fair to require balls not to be locked when that is part of the multiball modes... Also, even if that were true, you could still lock the balls to build up the Dracula letters until you only need 1 or 2, then unlock the balls, make the last shot, then relock them... Wouldn't be too much harder really... But yes, this aspect of multiball made the goal easier for sure. Found another video where the jackpot was earned with 2 balls locked:As i've said. i've gotten the 150 million jackpot (and thus not the goal) many many times, even with zero balls in the trough. every single time i've only gotten the 150 million, i've had balls locked for castle jackpot. the time i finally got it, i had three balls visible and moving around on the playfield.
either the game is trying to not count balls locked for castle jackpot among the three (which actually makes sense to me, making it so if you park balls you don't get the doubled jackpot), or three balls counts and 4 does NOT. the 300m jackpot in your video was exactly 3 balls in play. (2 in castle, one in trough, and one on field)