Connect your vacuum gauge again and monitor the rpm with something accurate too.
Then try turning your idle mixture screws a quarter turn lean.
If the rpm and vacuum increase keep going until you get a very slight drop in rpm and vacuum.
If the rpm and vacuum decreases when you go leaner then try going a quarter turn richer.
If going richer a quarter turn at a time makes the rpm and vacuum increase keep going until they stop increasing or start going down again.
Then back off lean until you get a very slight drop.
This will get your idle mixture spot on and your manifold vacuum as high as possible.
Also it is important that if you have an automatic that you do this with the car in Drive (and the brake on of course). Don't forget the car is in drive and then rev the engine like I did the other day. You get a fright doing that.
Again. With the car in drive you must make a note of the manifold vacuum. Use this figure to select the correct springs for the metering rods or Power Valve (Holley). Usuallly on the Holley you would rate the power valve at half the manifold vacuum. Can't remember how you do it on the Carter/TQ/Edelbrock. They have a chart or something.
If your RPM drops more than
(Edited) 300 revs when placed in gear after setting it then you need a higher stall torque converter.