how much initial timing mild 360 magnum

You're saying your dizzy has 24 degrees of mechanical advance? Or is your advance in the dizzy locked?

You're saying its all in by 2500, so I'm thinking you mean 24 degrees of mechanical advance.

I'd start at 12* initial without worrying about a start retard. I'm guessing yours will like 14 or 16 initial best. The magnum heads don't need as much timing as the older stuff because the chamber works better.

Probably however, you won't be able to tell the difference between 12 and 16 except at the track. Yes, all in by 2500 is a good place to start.

Steve