i always try to get as much vacuum at idle as possible, no matter how big the cam.
i do this by setting a base idle of around 900 or so, then tweaking the mixture screws until i get the most vacuum, as well as a higher idle reading. then i back the idle down to 900 again, then play with the mixture screws some more. keep doing this until it doesn't increase vacuum anymore by playing with the screws. remember, tho, that everytime you change the timing, you might need to do this all over again. but it really isn't that hard, and can be rather enjoyable once you get a good idle and crisp throttle response.
that being said, with big cams you aren't gonna be able to get 20" of vacuum at idle- it just aint gonna happen. my engine gets about 16 at idle, which seems pretty good just by looking at the specs of my gm HOT hydraulic roller cam- .525 lift, 219 intake duration, 228 exhaust duration @.050 lift. it has a slight lope at my 800 rpm idle, but not too bad.
1971 Nova(looks like 69 camaro from underneath!)
355sb, vortec heads, HOT cam,T-10 tranny, 3.70 gears 16X8" IROC wheels. 12" Corvette brakes on the way.
see pics here http://community.webshots.com/user/novaderrik