Depending on the overall condition of the engine, fuel, ignition systems... if you get it too low, it won't rev smoothly when you step on the gas.. even if it seems to idle fine. I've had some cars with bad carbs and/or lumpy cams that simply wouldn't idle well below about 1200rpm (a good bit too high)

if you can easily get the car going on level ground by letting the clutch out without applying gas.. the idle is probably too high.

800rpm should be about good. Indeed though, if you don't have the ear for that.. you'll just have to tinker. I wouldn't want my idle to be below about 650, but you'd be hard pressed to get it smooth below that anyhow.

Be sure to turn both idle screws equally to keep the carbs in sync.

--
-Matt I ♥ my ♂