So in theory, I could go one tooth off: 116-12=104
Then add 4 back in with the offset key way on the crank sprocket, and get where I need to be:
104+4=108
Something doesn't seem right. Things may not be perfect all the time, but let's check things out so you are not messing something up by trying to fix an issue.
What is the part Number of the timing set?
Can you show pics of the timing set with the numbers on the sprocket?
Also, btw, I'm liking your build progress.
Edited:
I'm sorry, I did not realize the magnums came with a chain tensioner. Maybe check your piston TDC dwell again and do the process again, just in case, it is always good to double check. If it's still 8° retarded, try as suggested and advance by a tooth on the sprocket and play around with the sprocket key slots to get close enough to spec.