The Importance of Needle Height

This example is from a Mikuni VM20 carb. This first red graph below shows a 5% variation from jetting perfection with the needle taper ready to start to have an effect when the carb slide is about midway between 1/4 and 3/8 slide open (about 30% open). At small slide openings anything within 5% is considered jetted really well.

This graph shows too rich at 1/4 to 1/2 slide opening due to the needle being two notches too high.

This graph shows too lean at 3/8 slide open due to the needle being two notches too low.

So this is why I recommend setting the needle height for good running up to 1/2 slide opening and then if any needle changes are necessary then try different needles with different tapers (after making sure the main jet is sized correctly for WOT running).
For Mikuni it's best to first set the needle height so that at 30% slide opening the beginning of the needle taper is right at the top of the narrow part of the jet for the needle. First take the needle out and put a black dot with a felt-tipped marker at the top of the needle taper. Find that by using a digital caliper to measure the diameter changes along the needle. Then put the needle in the slide and lower the slide in the carb till it has a 30% opening (engine side) and look for the needle mark. If it is higher than the narrow part of the jet then lower the needle. If it is below the jet so you can't see it then raise the needle.

The Keihin PWK35 I ran in the calculator needed the slide to be open 25% when the needle taper is at the top of its jet. A Dellorto VHSB34 needed only 19%. I'm not sure if these percentages are typical for all the carbs from each brand so it is best to use my jetting calculator to observe the graph to know where to set the needle before continuing to jet for mid range and top RPM. The average for the three was 25% which is 1/4 slide open.