You can set the idle at any speed above 550 rpm you want. Slower if you want, but most won’t run well below 550rpm. The engine doesn't care. It's how much float you get landing and how much brakes you use taxing. Most engines in the certified world, idle between 550 and 750 wherever the engine runs smoothest and accelerates best from.
Fuel pressure shouldn't vary much from idle to full power on engines equipped with dual diaphram pumps. If rotary pump equipped ilde FP and full power FP might vary as much as 20 psi depending on pump relief valve settings.
Good Luck,
Mahlon
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