I have a 385 (.30 over 383). If I remember correctly, it has either a 9 or 9.5:1 compression ratio (I forgot and have lost the build sheet). I have a 222*/232* @0.50 .447" lift cam w/ 111* LSA, 1.6 ratio roller rockers. The engine has mildly ported iron pro-topline heads w/ 180cc intake runners, 64cc combustion chambers, 2.02/1.60 valves. I have the performer RPM air gap intake w/ a 650 cfm vacuum secondary holley carb. What kind of vacuum should I pull?
I'm thinking of putting on a brake booster is why I ask, and I believe my vacuum may be a little low.

I would imagine more than enough to operate the power booster.
Any value over 10 inches would be good, I believe.

I was told by Tobin at Kore3 that he believed I needed 18 inches to run a stock 68 booster.

Your cam has 5 degrees of overlap at .050. With your 9-9.5 cr you should have plenty of vacuum for a booster. My friend has a mild 8.5 cr 350 with a cam that has 6 degrees of overlap at .050 He pulls 14" of vac. at 1000 and 11 at 800 and has no problem with his power brakes or with operating the vacuum headlights on his Vette. Higher compression always improves your vacuum, other things being equal. Also your cam in a 383 will seem milder than in a 350 and should pull better vacuum too.

I'm believe I have a leak or something. I think my vacuum was much lower than this - like 5" or something. I'll have more after I get to work on the car this weekend. My plan is to advance the timing as much as possible, make the car idle as low as possible, and see where it stands.

You should be in the teens if it's running good, no leaks etc.
My 406 has a cam with .250 @ .050 on 108 and it pulls 13" at about 900rpm, plenty for my stock booster. I wouldn't want to push my luck with repeated hard stops, but the pedal always feels pretty good.

On my old cam I had nps with 7"
Except if stopped, I took my foot of the brake pedal and put back on...it wasnt I didnt have brakes , they where just non boosted.
In practice u u use your brakes at the same time as the engine brake..Very high vac up around 22+ " or cruise again 16 to 20"

This cam I have about 14"

My plan is to advance the timing as much as possible
But do not over advance the total cent advance and total cent+vac advance

Subject covered in detail in several previous threads

My 383 with 230/236@ .050 has 16+ inches at 750 rpm-you should have plenty.

My ZZ383 pulls 10.5" at 800 rpm. I think it's somewhat low but belive that is due the fact that I'm at 5,500 ft. above sea level were atmosperic pressure is significantly lower. I put a vac cannister onto my booster system, have 4-wheel disc brakes and all works fine.

I would think you should have between 12-14 inches no problem.