How to fairly distribute resources when the recipient are not equal?

I am setting up a crowdfunding website for medical research projects. We've decided to have 5 research projects/campaign. People could make a specific or general donation and the team would distribute these donations fairly among the projects.

The projects are different in - their budgets - their scopes (different diseases) Another level of complexity is the ALL / NOTHING model. Let's assume the budget of a research project is 5000 USD. If the researcher gets 100 USD of donations, it is too small to make any difference. Typically it could be 1000, 1500 and 2500. So if the campaign raises 1500 USD, this could increase the chances to find one or 2 other grants to have the research done. The "all or nothing" threshold should be a fraction of the total budget, it should be the minimum amount of money that could make a difference.

Here is my proposition: Assuming: - 5 research projects per campaign, each with different sizes of budgets B. The total sum of these budgets is T - individual projects will be able to receive dedicated donations or a fair share of non-dedicated donations. - before they reach the mini-goal they can't take "home" the money they raised but the campaign goes on for 3 months

Proposition: (1) Before reaching the mini-goal: Dedicated donations are allocated to the dedicated projects. General donations G are shared between the projects proportionally to their budget:

Scenario (1): Each project will receive GB/T until all projects reach their respective mini-goal If one of them reaches early its mini-goal, it will continue to receive its dedicated donations but not general donations. Its B value is subtracted from T and now on projects receive GB/T, T having a new value. Until they reach their mini-goal.

Scenario (2): Each project will receive G*B/T until all projects reach their goal or the end of the campaign.

Do you agree? What scenario do you find fairer? Do you have another scenario to propose?

Thanks!