There is a major issue with the proposed scoring - it is underspecified. In particular, in the definition:

for B in your beliefs that's true

How do we determine if B is in an agent's set of beliefs? We cannot only consider the beliefs that are currently running through the agent's mind, because we'd end up with at most a few. We need a definition of what "B is in your beliefs" means. However, it is very difficult to specify all of an agent's beliefs - humans don't walk around carrying a well-defined sack of beliefs with probabilities attached.

Less importantly, the linearity in the sum can be exploited. For example, I can easily get myself to believe the following sequence of statements in Peano arithmetic:
1=1
2=2
3=3
...

This will give me a favorable score with minimal effort. At least in this case, the proposed scoring is orthogonal to measuring epistemic rationality.