How best to measure similarity between a corpus and a document?

I am making use of fine tuned llms for a project and hoping to have introduced specific literary styles to the their output. My professor would like me to quantify how much better they perform than a base model, what would be the best way of doing this when compared to a test corpus?

So far i'm thinking I can use various stylometric indicators (sentence length, vocabulary etc.) and maybe cosine similarity of tfidf vectors. Would this be the best …

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