Bear with me here, as we not only need to answer your question, but make sure no damage is done to the rifle.

The short answer is to start with the largest orifice, and work down in size until the rifle cycles reliably. But before you start, you might want to look into a few things.

If the provenance of the op rod spring is unknown, I strongly recommend you replace it with one of new manufacture. The spring in most surplus M1icon's is likely between 50-70 years old and has taken a set after use and being in compression all those years. A new spring will make the rifle shoot noticeably softer, and the action will be snappier. Most important, it will reduce the pounding the receiver takes when the bolt stops at the end of it's travel rearward. Don't forget to grease the spring with rifle grease.
Fulton Armory has new springs made to the original print by one of the original contractors.
Operating Rod Spring, New, Fulton Armory GI Spec

(I realize his may not help much as Fulton won't ship outside the US, but others may find it useful)

Ammo:
Privi Partizan makes two different 150gr FMJ loads. Under their military product line, they list M2 Ball. If this is the ammo you have, the standard plug will work fine. This would be the same as USGI M2 Ball that the rifle was designed to shoot.
Under their sporting ammo line they list another 150gr FMJ load that is loaded 100fps faster than GI specs. If this is the ammo you have, it would be prudent to use the adjustable gas plug.
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