Here are some images from a FIAT SAE Paper from circa 1987.
You can see how there are diminishing returns as the angle is lowered,then goes asymptotic at 64-degrees,with no further drag reduction beyond this angle.
Here is the FIAT drag curve for the cowl/windshield intersection that Frank was asking about.Again,the curve 'flatlines' at a particular angle,with no further benefit.
This lower image prompted the hood blister for the T-100.