On the subject of ignition timing there are a couple of things to keep in mind.

I think the big thing is what the total advance is. Checking the timing at idle is more of a reference point than an absolute. If I remember correctly the distrubutor reaches full advance around 2500 rpm, maybe a little higher. Full advance is usually in the 32 to 34 degree range.

I have always used the timing light to get in the ball park but did the final fine timing adjustments by ear. Basically did full power runs up a steep hill by my house. Advance the timing a small amount at a time untill you start to get some pinging. back off the timing enough to stop the pinging and call it set! After this procedure measure the timing at the specified idle speed and note what the setting is.

If you get pinging at low rpm only the springs in the distributor may have weakened and are allowing the mechanical advance to kick in too soon.

Hope this helps

Gary