Not a feeler gauge, but you should have around .020 exposed on primary and secondary. They will look like little squares. If you have to raise the idle speed a little, work the secondary first. If you have to go to much because of a large cam/low idle vacuum AND the making sure the timing is optimized, you will need to drill small holes in the throttle blades to add idle air without excessively uncovering the t-slots. Start with a 3/32 hole in the primaries, add secondaries if needed. Best position is directly below the idle discharge port as shown below. "X" marks the spot.
