How much would I personally have to reduce X-risk to make this the optimal decision?

Shouldn’t this exercise start with the current P(extinction), and then calculate how much you need to reduce that probability? I think your approach is comparing two outcomes: save 25B lives with probability p, or save 20,000 lives with probability 1. Then the first option has higher expected value if p>20000/25B. But this isn’t answering your question of personally reducing x-risk.

Also, I think you should calculate marginal expected value, ie., the value of additional resources conditional on the resources already allocated, to account for diminishing marginal returns.

I’m not sure I follow this exercise. Here’s how I’m thinking about it:

Option A: spend your career on malaria.

Cost: one career

Payoff: save 20k lives with probability 1.

Option B: spend your career on x-risk.

Cost: one career

Payoff: save 25B lives with probability p (=P(prevent extinction)), save 0 lives with probability 1-p.

Expected payoff: 25B*p.

Since the costs are the same, we can ignore them. Then you’re indifferent between A and B if p=8x10^-7, and B is better if p>8x10^-7.

But I’m not sure how this maps to a reduction in P(extinction).