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Yep, a policer usually has a detrimental (actual less than CIR rate) effect on traffic that's bursty (such as TCP) as you decrease the Tc.
Is there a formula? Not that I'm aware, but in theory, there could be. Performance should be deterministic, but there would be multiple variables when working with actual traffic. Some would be hard to answer such as the relationship of when bits are actually on the wire vs. where is the policer in its Tc measurement cycle. (For the last, consider a burst of bits, same volume, where all fall in one Tc interval vs. them spanning across two Tc cycles.)
