The "old" general rule was 1/2 the fan blade depth has to be outside of the fan shroud. The motor side of the shroud should be at, or close to, the center line of the spinning fan. More or less fan blade inside the shroud was considered as not as efficient.
that was what I'd gleaned from reading the multitude of threads on this topic, and that is how I adjusted the shroud extensions
71 Demon (project): 318, A-833od, 8-3/4, 3.23's
14 Wrangler JKU M-380 "Kilroy" (under construction): Magnum 360, 46rh, Atlas4, D60/14b-5.38s-Grizzlys, 40s
07 Ram 3500 MegaDually 4x4 "Big Mack": 5.9 6BT, G56 'Toy Hauler'
52 Willys M-38 "Poncho"
| 
