I recommend not distributing it to project members at all. Appoint or elect a treasurer, open an account and deposit the money to earn simple interest. If you distribute donated funds between developers, at least one will become disenfranchised as the project grows.
Instead, consider the other possibilities for the funds:
Swag. Print up some T shirts to send to people who have made more than trivial contributions to the code.
Bounties. Put a reward on the feature the community really wants but nobody really feels like implementing. This is also a great way to get new long term contributors. Or, 'sweeten the pot' a bit for fixing a really perplexing bug. The bounties don't have to be cash, especially if you have swag to give. It also doesn't have to be swag, a Pi in hand is worth 10 in the oven.
Hardware. Buy stuff that the community owns that all developers can use. This could be servers, or gadgets that are shared through the mail.
Tools/Licensing. You might need to pay for software, even being an open source project. You might need to buy a copy of Acme Widgets to study it for the purpose of creating an open alternative, or you might need to help a great contributor upgrade their development environment.
Events. Help send your developers to conferences or key events when opportunities present themselves. Or, sponsor your own meetup if enough people would be able to attend.
Legal Fees. While (thankfully) still relatively uncommon1, you might find yourself in litigation for a number of reasons. It's good to have some money put away. This could be simply enforcing your license or copyright, or defending against something else.
There are so many ways that the money could be spread so everyone benefits, it really doesn't make much sense to limit the good it could otherwise do.
If you get to the point that donations and community support make hiring some of the most prolific developers to work on the project full time reasonable, it means you should be looking at the project as more of a business than a hobby.
1 Litigation in open source is a lot like plane crashes. You read about the horror stories and drama in the news when something happens, but don't forget the nearly millions of projects that have and will continue to exist without any issue whatsoever