Fan, Shroud & Radiator - Optimal Spacing?

Hi all. Just wondering, does anyone have any experience in trying to find the optimal spacing btw fan blades & radiator both with and without shroud? Also, when fabricating a shroud, is there an optimal radiator to shroud depth and/or fan blade to shroud spacing? I'm sure there are many other variables to consider such as number of blades, clutches, blade pitch, etc that influence the outcome. But are there any rules of thumb out there?

Can ya tell, ...I'm an anal engineer!

Thanks in advance.