Think about it this way: the fee that you would otherwise get paid for your services (aka "fair market value) is being put at-risk. This is your "bet" on the future of the company. Your reward, if the company does well, will be a portion of the profits or a portion of the proceeds of a sale.
Your share of the rewards (%) should equal your share (%) of the risk.
If you and I went to Las Vegas and each bet a dollar on the same hand of Blackjack you would expect 50% of the winnings, if we won. This is because you took 50% of the risk.
If the dealer dealt two aces and I was broke so you put down $2 more you would no longer be satisfied with 50%. You bet $3 against my $1. You deserve 75% of the rewards, if any.
The same goes for startups.
All you do is determine the fair market value of each person's contribution. That's their bet. Your bet, relative to their bets, should be your % of the rewards.
This is the basic principle behind the Slicing Pie model, a form of dynamic equity split that guarantees a perfect split.
Every other model is a guess that depends highly on your ability to predict the future. This just lays the foundation for future disputes.
I've written a book about how this works and I would be happy to give you a copy if you contact me through SlicingPie.com