Spark advance questions

I play with spark now and then, but wanted some input/opinions from others with an M278 or M157, like how far have people have pushed spark at full load?
Also, has anyone raised the MBT/Optimum chart sky high to make the Target map more cooperative? As you can see I'm bumping MBT a little, but figured I'd ask before getting crazy.

Lastly, anyone know what the actual MBT would be for this eng? The oem MBT chart looks about what I'd picture MBT being, but the way the chart works tells me no? But what are the odds it's not, yet just happens to look like what I think MBT is? Very confusing. Basically I want to know what to shoot for, and I fear exceeding MBT, but I need to know where it is first...

The pix below is my spark now, oem, and the delta between them.
I'm running ~50% Ethanol which is why it's retarded at lower loads.

Attachment 161063

have you played with the low load retardation?

in my limited experience, not all applications prefer it

I don't know what that is, but I attached the oem and current tune if you want to look

Attachment 161329
Attachment 161330