I wouldn't assume that the ability to assign numerical probabilities would even be useful for epistemic rationality, since even if there were a way to calculate a consistent measure of 'degree of belief' it might be so computationally intensive that to give this number in any reasonable amount of time would require corner-cutting which would result in the final number being less than fully representative of your 'subjective probability' anyways.

(Granted: such an ability might be of aid to gamblers)