100% double spending attack probability When we plot the attacker's computational power against the probability of double spending with a number of blocks equal to zero, we always get a chance of 100%.

This plot is based on the original white paper that models the probability of success as a Poisson distribution.

Is this correct?

Shouldn't the probability of success then be related to the computational power the attacker possesses?
It is implicitly assumed that the attacker performs a Finney attack, and hence gets a block for free. If the merchant accepts a 0-conf tx, the attacker will proceed immediately to release a double-spending block he has already computed, and thus succeed.

In any case, Satoshi's analysis is approximate, see https://bitcoil.co.il/Doublespend.pdf for a more accurate one (which makes a similar assumption).