I have a 408 small block with a vacuum secondary Holley with two corner idle adjustment. I have hooked up a vacuum gauge to adjust for best idle and I get it at 1/2 turn from bottomed out. Does this seem right? Idk why it seems like this can’t be optimal. Thanks for any help.

 

Your symptom is because the primary plates aree further open than normal and allowing fuel from the main circuit rather than the idle circuit.
On the carb, pass side on the base plate is an adjustment screw for opening the secondary plates for air bypass, so the primary plates can be closed allowing the idle circuit to work as normal.
You can open and hold secondary plates full and measure height of the screw, as the slot is on the underside and carb removal is required for screwing 'in' the screw opening the secondary plates.
You can place a feeler gauge between the screw top and arm and add thickness to the measured distance, make primary adjustments, both idle speed and needle, for best results, then remove carb and adjust screw height, measured height and add feeler gauge thickness, and repeat process from there.
Also, placing manifold vacuum on distributor vacuum advance for advancing initial timing for better vacuum reading.
The more vacuum the better.