Service Manual says dizzy should be at 8 degrees ATDC w vacuum at 800 RPM. Because at cranking speed dizzy should be at 10 degrees BTDC wo vacuum. The amount of retardation is approximately 20 degrees because at 800 RPM a couple of degrees of mechanical advance is present. At no load w vacuum at 1500 RPM dizzy should measure at 0-5 degrees BTDC which according to the curve chart reflects approximately 10 degrees of mechanical advance while remaining under the influence of approximately 20 degrees of retardation.
This is where I get confused because the Service Manual states that w vacuum at 3000 RPM advance should be 25-30 BTDC. But there should be no vacuum retardation at 3000 RPM because the speed switch should have cancelled the retardation at about 2500 RPM. With no entries in the chart for higher RPMs I would infer from the chart that 25-30 BTDC is the highest resulting advancement. There is some logic here that makes sense to me. 10 degrees BTDC at cranking speed with 20 degrees of mechanical advance which tops out at 2000 RPM results in 30 degrees total advance at 3000 RPM.
Retardation at 800 RPM from 10 degrees BTDC to 8 degrees ATDC and at 1500 RPM from 20 degrees BTDC to 0 degrees.
When engine is cold or too hot then the engine is running in only mechanical advance with no retardation. 10 BTDC at cramnking speed. ~12 BTDC at idle, ~20 BTDC at 1500 rpm and ~30 BTDC at 3000 RPM. Retardation only subtracts the way I see it.