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Port = w_pa*A + w_pb*B, where w_pa is weight of A in portfolio, and
Market = w_ma*A + w_mb*B, which w_ma is weight of A in market portfolio (which is actually the solution, in another page, that dynamically solves for the A/B mix with the highest Sharpe ratio, so these are hard-coded in the linked XLS but they are solved in my XLS)
Then cov(w_pa*A + w_pb*B, w_ma*A + w_mb*B)
= w_pa*w_ma*cov(A,A) + w_pa*w_mb*cov(A,B) + w_pb*w_ma*cov(B,A) + w_pb*w_mb*cov(B,B)
= w_pa*w_ma*variance(A) + w_pa*w_mb*cov(A,B) + w_pb*w_ma*cov(B,A) + w_pb*w_mb*variance(B)
=w_pa*w_ma*variance(A) + (w_pa*w_mb + w_pb*w_ma)*cov(B,A) + w_pb*w_mb*variance(B) ... is the covariance(Port, Market) in my XLS. I hope that's helpful!