how much initial timing mild 360 magnum

You're confusing stuff here. You mention initial in the distributor of 24*, that's not initial, it's mechanical advance controlled by weights, springs and engine rpm.. Should have zero effect at idle.

Initial is the timing on the crank balancer. Set that first. Make sure it doesn't change when you pull the car in gear, that's BAD! Then determine how much mechanical advance you need to get the total number (total - initial).

LX, JD451 and 1wild have some good advice in here as well.

We used to run dual pickups in distributors to retard launch timing way back before electronics made it easy. Just what you needed, more buttons.switchesto flip as if you weren't busy enough. :)