No idea about the oil capacity but 2" or so from the filler neck would be a good starting point, and you can top up as required when it's running.
A "weak dribble" is a relative term, but it's always been a regular pulse in my old Trumpets.
Have you tried the old trick of putting your finger over the oil tank scavenge pipe with the engine running to force oil up to the rockers?
Then again, have you had this engine running satisfactorily in the past, or is it a new build?