On my 350/550, i just taper trim a section of tubing to twist into the bowl drain screw hole.
My 750's PD carbs allow me to place the tube on the overflow pipe, so no parts like this are needed.
It cant show you a float needle is leaking, but it will show you what your fuel level is.
If you used the factory tool to set the float heights (confident it is perfect) your fuel level should fall in that +/- 3mm below bowl top edge range.
If it doesn't, further investigation is needed
Check float pin smoothness.
Check float needle slides smoothly in its seat.
Check floats are not twisted on their hinge axis (one of the float lobes will measure differently than the other)
check for wear ring around tip of float needle preventing it from seating/sealing.
Compare all float needles for differences, like my picture above.