50% is usually for a longer term staking arrangement with makeup.  For a one time shot they usually get between 75% – 85% of the winnings.  This makes sense because of the variance tied into a 1 time deal.  The longer deal gives the backer more opportunities to recapture their money.  In the longer term deal you usually have makeup which means that before you can take any profit you have to clear your past buyins.  With a one time deal there is none of this.  They give you the money, you take the shot, and there is no further obligation.

The one time deal ratio doesn't change if you have more people.  Just break it into smaller pieces.  You can say for every 10% of buyin they get 8% of winnings and extrapolate from there.