Your query seems odd, because on the majority of the mowers (or ALL of them) I have seen, there is no way to adjust the blade/deck bottom relationship...are you thinking about making a custom deck, or perhaps thinking of using spacers to change the position of the blades in relation to the deck bottom? IMO having the blades within 2-3 inches of the lower edge of the deck works best as far as airflow is concerned...and that is probably where the makers design them.
Want to tell us why you are asking? I am in the mechanical trades and love to learn new ideas !!