Credit to Mumbles for finding this..
This is fine, if the float needle doesn’t have a spring-loaded plunger that contacts the float tab. If it does, then you can’t hold the carb fully upside-down, as the weight of the float will compress the plunger and make it appear as if the float is set way too high.
To gauge float height in those cases, only turn the carb far enough into the vertical that the float tab contacts the needle but doesn’t actually compress the spring plunger.