How to "score" partially decoded ciphertext?
You are looking for a way to measure the similarity between strings, which is usually refered to as string metric. Now in your case one of the strings is the original plaintext and the other is a (more or less) similar string generated by encrypting and decrypting only partially correct. But actually, it doesn't really matter where the other string comes from, you just need to measure how similar they are.

However, there are many string metrics, and it's possible to adapt them to your specific needs. Here are a few common ones:


Hamming distance: Number of mismatching characters
Levenshtein distance: Number of insert/delete/substitute operations


There are countless other ways to measure the similarity between strings, and it has to be determined what is considered similar and what isn't.