""If I adjust the idle mix screws what vacuum number am I trying to achieve?""

You are trying to achieve the highest vacuum reading at the N (since you have a 4 speed)rpm that you desire. If you want it to idle at 950rpms, than you adjust your mixture screws to get the highest vacuum at that rpm. As you get closer to the best vacuum number, your idle will increase, so you lower it back down and keep adjusting. Before you start the engine, count how many turns out each one of your mixture screws are set at. Then set them both to the same setting and hook up a vacuum gage so you can see it from the engine area. Then once the engine is warmed up and the choke full open, then turn one of the screws in or out (your choice) 1/4 turn and watch what the gage does. If the gage goes up then you know you are going in the right direction, if it goes down, then turn it back 1/4 turn to where you started and then go 1/4 turn the opposite direction. Once you know which direction you need to turn the screws, then wait 30 seconds or so before making another adjustment. Go to the other screw and do the same adjustment. You have to keep going back and fourth until you get the best vacuum reading and keep readjusting the idle rpms back down as needed. As you are adjusting each side, you will get to a point where you can only adjust each screw by 1/8 of a turn or so to keep the highest reading, then you may need to adjust them even less to maybe a 1/16 of a turn to get the best vacuum at your idle rpms. Don't be afraid to take your time and do this back and fourth for a while. Once you take the time to do it right, then you should have your highest reading and maybe a steady vacuum reading pending on your setup. Then you figure out what power valve to run by dividing this number in half to start with.