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Howard
Doc F has target flow rates for given volumes and duties - fans are sized on meeting that target, plus the pressure drop in the length of ducting, plus other margin factors. So, the right size fan pushing air through a correctly sized and installed duct, with sufficient inlet air via leaky door, will do a very good job of extraction - it's just most tradespeople will say - "bathroom, 4" fan, loop a bit of flexi-duct over the rafters (where it'll sag and become a blockage to efficient flow), external grille, jobs a good un"