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BFT gives you 20% CDR. Masteries give at least 4%.
So, your base should be at least 24% CDR. Some champs I'll build Ionian for 15%, and on others I'll go Sweeper for 10% and run Sorc boots (this is usually for assassins since your job is to knock out APCs and ADCs, neither of which build high levels of MR (so sorc boots are more effective).