I was having trouble tracking down an answer for this questions a while ago for my '77 F250. It's not in ANY of the shop manuals, as they all say "check the sticker under the hood" - well my truck didn't have that sticker anymore!
I set the initial timing at 10* BTDC (Vacuum unplugged). I probably could've gone higher, but the truck was running 87 octane
Now that I'm on 93 pump gas, I could probably bump it up a little. (maybe 12*).
At full mechanical advance, I think I saw around 35* Total advance (which would mean the previous owner put a 13T centrifugal unit in for 26* of advance). My distributor comes in with full mechanical advance at around 1,500rpm, which is probably a little too soon. The previous owner probably modified it. He did a lot of things the "cheap way" if you get my drift
With full manifold vacuum on the distributor, I also saw around 30-35* of advance, I may need to mess with the vacuum canister (and also mark my crank pulley for more accurate timing, it only goes from 0* to 30* advance, and you can't read any of the marks except for 10, 20, and 30!)
Does this help? I've seen answers varying from 6-12* BTDC for static initial timing. These answers are probably based on lots of factors, as what kind of fuel you run, what altitude you're at, and overall mixture control.