I was told at idle you should be around 6 to 10 degrees btdc, and at full advance no more than 34-36 degrees. Maybe someone else here can be more accurate than me, but this should be close.
imho.... as a rule of thumb, you want to get as much advance in as soon as you can, without ping.
I personally run almost 20 at idle, 30 at about 2000 and 38 at redline.
That is on a motor with alloy heads though.... (and well pollished chambers at that)
On an iron motor, probably 2 to 4 less at any given point, depending on gas quality, chamber size, plug heat range, etc...
Just an opinion.