When I had my Seery, it was set at 20 mm and it worked well for me.
Though, I imagine that there are variations in any flute model from one to the next, and also how it is blown by the individual-so you probably need to see how it works for you wherever you set it.
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A good starting point for the tuning cork is one head tube diameter back from the center of the embouchure. This will be around 19mm or just under 3/4 inch for most flutes. It is worth buying a 5/8 inch diameter dowel from the hardware store and making marks from 15 to 25 mm from one end (or 5/8 to 1 inch depending on your preferred units). You can use the dowel to move the cork and measure its position. The final adjustments should be made by playing and checking the tuning between the first and second octaves (assuming you will not be playing much in the third octave). If the upper octave is flat relative to the lower, then move the cork towards the embouchure a mm or so and try again. If the upper octave is sharp, move the cork away from the embouchure.
The cork only makes minor difference to the low octaves, so use the slide to get best general low octave tuning with the cork at the nominal (19mm) position. Check the relationship of the three D's and make adjustments to corkposition to optimise that. Readjust slide if necessary afterwards to correct any changes in the low octave (unlikely). Now check the tuning over the range of interest (the first two octaves or more if you use more). You may need to readjust the corkposition if it has created any problems there. You might also need to readjust it if overall response is weakened.
Kevin Krell
International Traditional Music Society, Inc.
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Wooden Flute Obsession CDs (3 volumes, 6 discs, 7 hours, 120 players/tracks) https://www.worldtrad.org
