It sounds as if you've hit a sweet spot with your timing so if the engine runs and performs well I think I would leave it as it is. It could be that the lighter spring in your distributor is a bit tired and allows a little bit of extra advance at idle. Hard to find a new spring maybe but the attachment peg can be tweaked a bit to add a small amount of extra tension to limit the advance at idle.
The advance until it pings and then move back is a well tried method but as you are already above the spec for advance I would leave it. In fact I would come back a degree at a time until there's problems and then advance until they disappear. Running the best fuel you can get is best for these engines as they were spec'd to use the European 100 octane so do need a bit of compromise now with modern gas.