There is far too much going on at idle for there to be any single answer.
Header surface area, compression, header thermal conductivity, cam overlap, flow restriction, mixture, fuel consumption, and ignition timing all have a large influence.
Street driving my car without excessive under hood pipe and turbo heat required a stoichiometric mixture and around 28-30 degrees advance. The main thing I worried about was all the heat from the pipes and turbo.
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89 LX. 363, single turbo, Super Vic EFI, TFS high port heads by TEA, solid roller, glide. Holley HP EFI. (exact combo varies)