Seems like you could add a variable to change the ratio based on how many people sign up too.
x = starting amount
b = bonus amount
n = number of players entered
c = constant to divide the number of players

1) First pond:

c = b X n ÷ x
c = 500 X 249 ÷ 20000
c = 6.225

2) Czuch's 19000 first bid pond, makes x = 1000
Note there's also less players in this game, but the bonus is the same.

c = 500 X 16 ÷ 1000
c = 8.000

If this has any correlation to how the game is played, then what Czuch has done (by design or not), is modify the original Pond game to make a smaller version that should play near the same. Though the bonus is going to figure hugely in the game from the start as it is now slightly more than ½ the amount of points the leaders have.
It seems to me that the most important variable of the game is the amount of players in relation to the ratio of bonus to starting amount. It'll be interesting to see how these two games play out. I'm in them both and will see first hand. Of course the various people involved and what happens along the way will make the game what it is.
Perhaps the game's starting point could be based on the number of players signed up using a constant of the game creator's chosing. I kind of like the table Bry and BigBadWolf have thought up. It'd be easy to look at the table and pick the values that one would like. As we get more experience playing this game, we should be able to make choices the will give us the kind of game we would like to play. A slow calculating game or a fast gambling game. Use of a table along with a variable for the number of players and you could have it set how you want and not need to bother with capping the amount of players except to limit the number of days a game will take to play.