Racecarl,

I don't know how much initial advance the mark yields. In this case one would have to rely on performance rather than a "number".

I think if you advance your timing enough you will get preignition or ping.

There are two indicator of excessive advance..increased operating temperature and/or pinging.

You should be able to use one, or both to set your timing to the safest maximum setting for best engine performance.

Stuart