The simplest formula I know that is based on number of participants is:
n - r + 1
n is the number of players
r is the player's rank
This gives 1 point to the first player to bust out, and one additional point to each subsequently busted out player. I don't really know how to incorporate your "Fantasy Buy-in". Is the Fantasy Buy-in the a pretend amount that it costs to enter the tournament? So some tournaments could (were they not free) cost more to enter than others, and thus should be weighted differently as far as points outcome? If that's the case, you can set the buy-in amount on the Game tab, even though no one really pays it to enter the tournament, then reference the buy-in with the bc variable (buy-in cost). But how you would like to incorporate that is a different matter. How much should it influence points?
As far as the Poker Leaderboard plug-in, as long as you use the corresponding export to export your tournament, your formula shouldn't make any difference.