best initial timing: how to find. need guidance.

[dusterbd13]

small block chevy. unknown compression ratio. unknown cam. automatic trans with 3.42 gears. weiand dual plane intake, 600 holley vacuum secondary, factory HEI out of god knows what. chevelle manifolds and 2.25 dual exhaust.
since we don't know much about this thing, I have no idea what to set initial timing to. we have no mechanical or vacuum advance acting at idle, so were talking pure initial timing.
what methods are there to determine best initial timing? I was honestly thinking of getting the thing to idle as low as possible, and hook up a vacuum gauge. keep cranking in timing until the manifold vacuum starts to drop off, then backing it off 3-4 degrees. what is wrong with this approach? I know the rest of my timing curve will be all sorts of out of whack after that, and I will address that by the same means I always have: dial back timing light to get an idea of total timing, figure out how much total mechanical it can stand before detonation on junk gas/heavy load/high throttle, and then figure out where the curve is happy. then add in as much vacuum advance as I can.
but this post is about initial timing, and how to determine where to set it. please advise. wanna get this done and get the ignition started to be sorted prior to getting the new carb dialed in. currently its at 14 degrees btdc, and starts like theres barely any advance in it. (unlike my mopar that has 12, and will kick back on the starter hot)
Michael