RE: low needle speed question?
When there is a starting point, it's for a specific engine. And you'll find it in the engine's manual.
Very seldom is that given by a mfg. nowadays. Most of them do not have tolerances that deliver such repeatability. And it's often that they change things on an engine that would invalidate manuals that are that specific.
But you can count on one thing. A general rule for all engines that'll work for yours will work or not based on nothing other than chance.
BTW, the picture of the engine shows a carb with an airbleed setup.