There is a not-too-old British book (I am not at home to take a picture) and the author categorized different combustion chamber designs (many of them every version of primitive open chamber) with corresponding initial and advance timing degrees.
Now I am just a keyboard warrior as inkjunkie would say, but with different inefficiencies in the combustion chamber, the initial advance gets larger-as I understand it.
Let the lords of wisdom correct me if I am wrong.