I'd say around 10in/hg at idle and below.. that should keep the fueling stable and consistent.. basically it should maintain a solid MAF frequency for the stock ECU and it should run smoothly.. once you get that, you can fiddle with the idle speed setting again to get it where you want, once you do that, alter your fueling in those cells so your fuel trims remain as close to 0 (no more than +/-10%) and the car should idle steady.