A damped vacuum gauge is best, but the MAP reading will do if a proper needle-type analog gauge isn't available. The goal is to run enough timing advance that you obtain the highest manifold vacuum level at the lowest engine speed possible. The optimum advance can only truly be found using a dyno, but another "cheater" method is to watch manifold vacuum levels. Wild, lumpy cams may require 24-30 degrees depending on the compression ratio, cylinder head port sizes, etc. For medium cams, try within the range of 18-22 degrees of advance. For mild cams, an idle ignition timing advance of approximately 16-18 degrees will do. If you have the air/fuel ratio set properly and your engine can idle somewhat decently, even if it's by holding the throttle pedal down off closed-throttle to keep the engine from stalling, you can begin by setting a ball-park ignition advance value.
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