I've been looking at 60cm cooker hoods and found that many have 120mm outlets, recommending not to reduce else cause a reduction in performance. Reducing from 120 to 100mm is a 31% loss of cross-sectional area. Looked at Wickes, screwfix, toolstation and no 120mm flexi ducting to go with their 120mm outlets! There's lots of 100/150mm ducting. Now, I have managed to find some 120/125mm ducting (inc B&Q), but why don't the suppliers of the hoods sell 120mm ducting to go with their hood recommended size? Is this an anomaly that others have noticed or am I missing something?