You should just be discounting expected utilities by the probability of the claims being true, and then putting all your eggs into the basket that has the highest marginal expected utility per dollar, unless you have enough resources to invest that the marginal utility goes down.

Where are the formulas? What are the variables? Where is this method exemplified to reflect the decision process of someone who's already convinced, preferably of someone within the SIAI?

That is part of what I call transparency and a foundational and reproducible corroboration of one's first principles.

Leave aside SIAI specific claims here. The point Eliezer was making, was about 'all your eggs in one basket' claims in general. In situations like this (your contribution doesn't drastically change the payoff at the margin, etc) putting all your eggs in best basket is the right thing to do.

You can understand that insight completely independently of your position on existential risk mitigation.