You can probably find online how many turns out on the idle mixture screws on the carb would be good to start with and probably just leave it there. A Holley 650 or so you can start with all of them 2 or 2 and a half turns out and you will be fine. Then just adjust the throttle idle screw to where you like it, around 800 or so, and make sure it doesn't die when it is in drive with the brake on, around 600 rpm or so. If you have to go up a little that's fine, but if it keeps dying then you may have a vacuum leak somewhere or something wrong with the carb.

The screw you mentioned you didn't know what it does looked like the idle adjustment for your throttle position. The idle mixture screws on the carb needs to be adjusted first and then adjust your idle with that other screw after you get the idle mixture screws good for the most vacuum you can get.