So for example, if you have to pedal the throttle to keep the engine running at 750 RPM and you have 16 degrees Base Idle Advance with 70 kPa manifold pressure and you're holding the throttle at 22%, changing the Base Idle Advance to 19 degrees may result in 68 kPa manifold pressure with you only holding 20% throttle to maintain the same 750 RPM idle speed.
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.
