I would always advise following the MI, but generaly they are over engineered recomendations.
But considerating should be taken in to the amount of extraction you require and the distance of extraction to grille.
Normally a cooker hood ex-fan would come with the 150mm outlet, so would need reducing down if you are considering that.
Is there a reason why you do not want to or the kitchen fitter does not want install fit 125mm/150mm duct?
If it is only a short distance, reducing should not compromise the efficiency greatly. But if you already have 150mm/125mm reducer, seems pointless purchasing another reducer to take it down to 100mm. And by using 125mm you are conforming to the MI.
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