need tuning help on 340

Idle is too vague when talking about setting timing.
The 12* at idle will work when you get the engine idling around 650 - 700 rpm.
14 - 16* at idle is good if the engine is idling higher than that, say 750 - 950, which often is the case when first getting an engine running.
Both assume a stock or close to stock distributor setup.