15-20 and after it fires, bring it up to break in RPM and set timing at 35-38 and let it run. It's not going to ping with no load on it at 38* and will keep exhaust temps down.
Worry about what the operating environment timing events are after you get it broken in.
Agreed, this is a great ballpark figure.
My 360 motor has 23 initial, total mchanical of 35 by 2000 RPM, but a loose converter (4k stall) and 4.10 gearing which helps as well.
Good starting point though, then go tuning after your break-in is done, good luck!