Fluorocarbon leaders on braided line

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As a general rule when using fluorocarbon line as a leader on braided line how do you determine what the poundage rating should be? For example, 50# Braid use ?# fluorocarbon. Is there a percentage applied that could work, i.e., 50# x 80 percent = 40#? I am looking for the maximum strength fluorocarbon that would break before the braided line.