I have deceided that about .280 may be a valid starting point. MECOA sells a spigot Venturi with a 5 mm bore.
I was avoiding this thread (since as far as I know there is no such thing as a HP 40 Stunt...) but .280 with which spraybar? 5 mm with what size spigot? I don't know what size the HP spraybar might be. .280 is probably too big for a Silver Star with an ST spraybar, and way too big for a Gold Cup. With an OS or OS-copy spraybar, its vastly too large for either engine. .197/5mm is way too big for either one with flush outlet, but a large spigot with a long projection could be in the ballpark, with the typical 1/16 diameter spigot with .030 projection, still far too large.
It also depends on what sort of prop you are planning on using, large high-pitch props will tolerate less venturi than small low-pitch props.
I would always start with the very low end of venturi size and work up, not start out too big and work down. That means a starting choke area of .0170-.0175 square inches or so, I think.
Brett
p.s. by the way, what did you "research" to come up with these sizes? I am not aware of any serious efforts to use these engines, unmodified.